Firstly get the sources. An easy way is to simply FTP them. I got the NetBSD 6.1.2 sources from here. These files are all tarballs, so they'll need to be extracted.
Once the sources are extracted, it's necessary to build the toolchain. This toolchain can be used to cross-compile for any architecture supported by NetBSD, however I decided to build for i386.
cd usr/src/ ./build.sh -m i386 tools
Once you've done this you can simply build a kernel:
./build.sh -u -m i386 kernel=GENERIC
Or, you can build a release which includes the userland:
./build.sh -U -u -m i386 release
The completed binaries will be in usr/src/obj/