Building puppy is easy.

First you need to have a working unslung development environment.

Retrieve the unslung sources from CVS:

 cvs -d:pserver:anonymous@nslu.cvs.sourceforge.net:/cvsroot/nslu login
 cvs -z3 -d:pserver:anonymous@nslu.cvs.sourceforge.net:/cvsroot/nslu co unslung

Change into the unslung working directory:

 cd unslung

Build the toolchain, which includes the crosscompiler and other tools. This will take quite a while and will download a large amount of data, but is only required once.

 make toolchain

I found that make toolchain failed with the version for CVS above, but succeeded using the instructions at http://www.nslu2-linux.org/wiki/Optware/AddAPackageToOptware.

All that remains, is to build puppy:

 make puppy-ipk

You can then copy the resulting puppy package to your slug:

 scp builds/puppy_1.6-1_armeb.ipk root@slug:puppy_1.6-1_armeb.ipk

and install it:

 ipkg install puppy_1.6-1_armeb.ipk