This assumes that the package is already part of the NetBSD Package System. If it is not and you want to make it ready for pkgsrc,, then you are advised to read part II of this document, "Package Constructor's Guide".
To get the package source going, you need to get the pkgsrc.tar.gz file from ftp://ftp.netbsd.org/pub/NetBSD-current/tar_files/pkgsrc.tar.gz and unpack it into /usr/pkgsrc.
As an alternative, you can get pkgsrc via the Software Update Protocol, SUP. To do so, make sure your supfile has a line saying "release=pkgsrc" in it, see the examples in /usr/share/examples/supfiles, and that the directory /usr/pkgsrc does exist. Then, simply start ``sup -v /path/to/your/supfile''.