First let's find a possible candidate to package...

 http://nyancat.dakko.us/

Ok, it's seem that it uses GitHub, let's go to the Git repository and look at
the `Releases' page:

 https://github.com/klange/nyancat/releases

Allright! Let's try to grab the source directly via url2pkg:

 $ cd pkgsrc/local/
 $ mkdir nyancat
 $ cd nyancat
 $ url2pkg 'https://github.com/klange/nyancat/archive/1.5.1.tar.gz'

Now we can see the following via our favourite ${EDITOR}:

 # $NetBSD$
 
 GITHUB_PROJECT= nyancat
 DISTNAME=       1.5.1
 CATEGORIES=     local
 MASTER_SITES=   ${MASTER_SITE_GITHUB:=klange/}
 DIST_SUBDIR=    ${GITHUB_PROJECT}
 
 MAINTAINER=     pkgsrc-users@NetBSD.org
 HOMEPAGE=       https://github.com/klange/nyancat/
 COMMENT=        TODO: Short description of the package
 #LICENSE=       # TODO: (see mk/license.mk)
 
 # url2pkg-marker (please do not remove this line.)
 .include "../../mk/bsd.pkg.mk"

...after we exit from the ${EDITOR} we can see:

 url2pkg> Running "make distinfo" ...
 => Bootstrap dependency digest>=20010302: found digest-20160304
 WARNING: [license.mk] Every package should define a LICENSE.
 => Fetching nyancat-1.5.1.tar.gz
 Requesting https://github.com/klange/nyancat/archive/1.5.1.tar.gz
 Redirected to https://codeload.github.com/klange/nyancat/tar.gz/1.5.1
 Requesting https://codeload.github.com/klange/nyancat/tar.gz/1.5.1
  15124      141.59 KiB/s
 15124 bytes retrieved in 00:00 (141.42 KiB/s)
 url2pkg> Running "make extract" ...
 => Bootstrap dependency digest>=20010302: found digest-20160304
 WARNING: [license.mk] Every package should define a LICENSE.
 => Checksum SHA1 OK for nyancat-1.5.1.tar.gz
 => Checksum RMD160 OK for nyancat-1.5.1.tar.gz
 => Checksum SHA512 OK for nyancat-1.5.1.tar.gz
 ===> Installing dependencies for nyancat-1.5.1
 => Tool dependency checkperms>=1.1: found checkperms-1.11nb1
 ===> Overriding tools for nyancat-1.5.1
 ===> Extracting for nyancat-1.5.1
 tar: Global extended headers posix ustar archive. Extracting as plain files. Following files might be in the wrong directory or have wrong attributes.
 url2pkg> Adjusting the Makefile.
 Remember to correct CATEGORIES, HOMEPAGE, COMMENT, and DESCR when you're done!
 
 Good luck! (See pkgsrc/doc/pkgsrc.txt for some more help :-)

...and relooking at the Makefile we can now do some improvements and
simplifications:

 # $NetBSD$
 
 DISTNAME=	nyancat-1.5.1
 CATEGORIES=	games
 MASTER_SITES=	${MASTER_SITE_GITHUB:=klange/}
 
 MAINTAINER=	pkgsrc-users@NetBSD.org
 HOMEPAGE=	https://github.com/klange/nyancat/
 COMMENT=	Nyancat in your terminal, rendered through ANSI escape sequences
 LICENSE=	modified-bsd

 .include "../../mk/bsd.pkg.mk"

...and add a DESCRiption:

 $ echo "Nyancat in your terminal, rendered through ANSI escape sequences." > DESCR

Let's try to build it!:

 $ make
 => Bootstrap dependency digest>=20010302: found digest-20160304
 => Checksum SHA1 OK for nyancat-1.5.1.tar.gz
 => Checksum RMD160 OK for nyancat-1.5.1.tar.gz
 => Checksum SHA512 OK for nyancat-1.5.1.tar.gz
 ===> Installing dependencies for nyancat-1.5.1
 => Tool dependency checkperms>=1.1: found checkperms-1.11nb1
 ===> Overriding tools for nyancat-1.5.1
 ===> Extracting for nyancat-1.5.1
 tar: Global extended headers posix ustar archive. Extracting as plain files. Following files might be in the wrong directory or have wrong attributes.
 ===> Patching for nyancat-1.5.1
 ===> Creating toolchain wrappers for nyancat-1.5.1
 ===> Configuring for nyancat-1.5.1
 => Checking for portability problems in extracted files
 ===> Building for nyancat-1.5.1
 --- all ---
 cd src && /usr/bin/make all
 --- nyancat.o ---
 gcc -O2 -c nyancat.c
 --- nyancat ---
 gcc  -O2 -Wl,-R/usr/pkg/lib nyancat.o -o nyancat
 => Unwrapping files-to-be-installed.

Ok, it seems that everything properly built, let's try to install it!:

 $ make install
 => Bootstrap dependency digest>=20010302: found digest-20160304
 ===> Installing for nyancat-1.5.1
 => Generating pre-install file lists
 cd src && /usr/bin/make all
 install src/nyancat /usr/bin/nyancat
 install: /usr/bin/nyancat: open: Permission denied
 *** Error code 1
 
 Stop.
 make: stopped in /tmp/pkgsrc/local/nyancat/work/nyancat-1.5.1
 *** Error code 1
 
 Stop.
 make[1]: stopped in /usr/pkgsrc/local/nyancat
 *** Error code 1
 
 Stop.
 make: stopped in /usr/pkgsrc/local/nyancat

Whoops! The package doesn't seem to properly support DESTDIR, probably something
wrong is missing on the upstream Makefile, let's give it a look to it via pkgvi:

 $ pkgvi /tmp/pkgsrc/local/nyancat/work/nyancat-1.5.1/Makefile

Indeed we can see:

 install: all
         install src/nyancat /usr/bin/${package}
         gzip -9 -c < nyancat.1 > /usr/share/man/man1/nyancat.1.gz

Let's fix it (trying to make this change also upstreamable), we change the
previous install target to:

 [...]
 install: all
        ${INSTALL_PROGRAM} src/nyancat ${DESTDIR}${PREFIX}/bin/${package}
        ${INSTALL_MAN} nyancat.1 ${DESTDIR}${MANDIR}/man1/nyancat.1

...and on the top:

 [...]
 distdir = $(tarname)-$(version)

 PREFIX?=       /usr
 MANDIR?=       ${PREFIX}/share/man
 
 INSTALL_PROGRAM?=      install -m 755
 INSTALL_MAN?=          install -m 644
 [...]

...and when we exit we can read:

 pkgvi: File was modified. For a diff, type:
 pkgdiff "/tmp/pkgsrc/local/nyancat/work/nyancat-1.5.1/Makefile"

We can now generate the patches via mkpatches:

 $ mkpatches
 $ mkpatches -c

Add a little comment to the patch and generate the corresponding checksum for
distinfo:

 $ make mps

Ok, now we still need to make some little adjustements to Makefile:

 $ $EDITOR Makefile
 [...]
 LICENSE=        modified-bsd
 
 MAKE_ENV+=      INSTALL_PROGRAM=${INSTALL_PROGRAM:Q}
 MAKE_ENV+=      INSTALL_MAN=${INSTALL_MAN:Q}
 MAKE_ENV+=      MANDIR="${PREFIX}/${PKGMANDIR}"
 
 INSTALLATION_DIRS=      bin ${PKGMANDIR}/man1
 
 .include "../../mk/bsd.pkg.mk"

Let's clean it and try again to (re)install it:

 $ make clean
 ===> Cleaning for nyancat-1.5.1
 $ make install
 => Bootstrap dependency digest>=20010302: found digest-20160304
 => Checksum SHA1 OK for nyancat-1.5.1.tar.gz
 => Checksum RMD160 OK for nyancat-1.5.1.tar.gz
 => Checksum SHA512 OK for nyancat-1.5.1.tar.gz
 ===> Installing dependencies for nyancat-1.5.1
 => Tool dependency checkperms>=1.1: found checkperms-1.11nb1
 ===> Overriding tools for nyancat-1.5.1
 ===> Extracting for nyancat-1.5.1
 tar: Global extended headers posix ustar archive. Extracting as plain files. Following files might be in the wrong directory or have wrong attributes.
 ===> Patching for nyancat-1.5.1
 => Applying pkgsrc patches for nyancat-1.5.1
 => Verifying /usr/pkgsrc/local/nyancat/patches/patch-Makefile
 => Applying pkgsrc patch /usr/pkgsrc/local/nyancat/patches/patch-Makefile
 Hmm...  Looks like a unified diff to me...
 The text leading up to this was:
 --------------------------
 |$NetBSD$
 |
 |Honor DESTDIR and properly install the program and man page.
 |
 |--- Makefile.orig      2015-11-24 05:05:00.000000000 +0000
 |+++ Makefile
 --------------------------
 Patching file Makefile using Plan A...
 Hunk #1 succeeded at 3.
 Hunk #2 succeeded at 39.
 done
 ===> Creating toolchain wrappers for nyancat-1.5.1
 ===> Configuring for nyancat-1.5.1
 => Checking for portability problems in extracted files
 ===> Building for nyancat-1.5.1
 --- all ---
 cd src && /usr/bin/make all
 --- nyancat.o ---
 gcc -O2 -c nyancat.c
 --- nyancat ---
 gcc  -O2 -Wl,-R/usr/pkg/lib nyancat.o -o nyancat
 => Unwrapping files-to-be-installed.
 ===> Installing for nyancat-1.5.1
 => Generating pre-install file lists
 => Creating installation directories
 cd src && /usr/bin/make all
 /usr/bin/install -c -s -o leot -g users -m 755 src/nyancat /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/bin/nyancat
 /usr/bin/install -c -o leot -g users -m 644 nyancat.1 /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/man/man1/nyancat.1
 => Automatic manual page handling
 => Generating post-install file lists
 => Checking file-check results for nyancat-1.5.1
 ERROR: ************************************************************
 ERROR: The following files are in /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg but not in the PLIST:
 ERROR:         /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/bin/nyancat
 ERROR:         /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/man/man1/nyancat.1
 *** Error code 1
 
 Stop.
 make[1]: stopped in /usr/pkgsrc/local/nyancat
 *** Error code 1
 
 Stop.
 make: stopped in /usr/pkgsrc/local/nyancat
 
OK! We've got the above error because the PLIST is still not populated:

 $ make print-PLIST >PLIST
 $ cat PLIST
 @comment $NetBSD$
 bin/nyancat
 man/man1/nyancat.1

Let's try again to install it (in order to not rebuild it again we can use the
`install-clean' target):

 $ make install-clean
 $ make install
 => Bootstrap dependency digest>=20010302: found digest-20160304
 ===> Installing for nyancat-1.5.1
 => Generating pre-install file lists
 => Creating installation directories
 cd src && /usr/bin/make all
 /usr/bin/install -c -s -o leot -g users -m 755 src/nyancat /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/bin/nyancat
 /usr/bin/install -c -o leot -g users -m 644 nyancat.1 /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/man/man1/nyancat.1
 => Automatic manual page handling
 => Generating post-install file lists
 => Checking file-check results for nyancat-1.5.1
 => Checking for non-existent script interpreters in nyancat-1.5.1
 => Checking file permissions in nyancat-1.5.1
 => Checking for missing run-time search paths in nyancat-1.5.1
 => Checking for work-directory references in nyancat-1.5.1
 => Creating binary package /tmp/pkgsrc/local/nyancat/work/.packages/nyancat-1.5.1.tgz
 => Becoming ``root'' to make su-real-package-install (/usr/pkg/bin/sudo)
 ===> Installing binary package of nyancat-1.5.1
 ===> Installing for nyancat-1.5.1
 => Generating pre-install file lists
 => Creating installation directories
 cd src && /usr/bin/make all
 /usr/bin/install -c -s -o leot -g users -m 755 src/nyancat /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/bin/nyancat
 /usr/bin/install -c -o leot -g users -m 644 nyancat.1 /tmp/pkgsrc/local/nyancat/work/.destdir/usr/pkg/man/man1/nyancat.1
 => Automatic manual page handling
 => Generating post-install file lists
 => Checking file-check results for nyancat-1.5.1
 => Checking for non-existent script interpreters in nyancat-1.5.1
 => Checking file permissions in nyancat-1.5.1
 => Checking for missing run-time search paths in nyancat-1.5.1
 => Checking for work-directory references in nyancat-1.5.1
 => Creating binary package /tmp/pkgsrc/local/nyancat/work/.packages/nyancat-1.5.1.tgz
 => Becoming ``root'' to make su-real-package-install (/usr/pkg/bin/sudo)
 ===> Installing binary package of nyancat-1.5.1
 => Dropping ``root'' privileges.

Finally it seems to be correctly installed and we can try it:

 $ nyancat