Use scr2pkg if you want to install something in Slackware that's not on slackbuilds.org.
You can download a Slackware package from [ update: google it, it's bound to be maintained somewhere ]. After installation you need to run as root
src2pkg –setup to complete the installation.
src2pkg is unbelievably smart and gives helpful feedback if something is off. The default command is something like
src2pkg -CWD foo.tar.bz2. A tutorial is also on the src2pkg website.