Compiling is an option for you, definitely. I've actually never done it the way you're trying to do, I've always compiled my own kernel.
The sticky in this forum will pertain to any 2.4.x kernel, and is relatively the same for 2.6.x as well.
Be warned, though -- compiling on that machine will take quite a while, and it may take you a couple tries to get everything right.
A better option for you may be to install the kernel via slack-pack... ie the tgz
and then grab matching kernel source. Slackware uses the vanilla source, so you'd just grab linux-2.4.25.tar.bz2 from
www.kernel.org to match the latest slack pack for the 2.4.25.
Good luck
--Shade