Just install the source package like any other. You want one of the "kernel-source" packages. Do a little reading and determine what version you want. I got one of the 2.4 kernels. While you are at it, you want to get a few other packages that do all the work for you. Here's the list:
I use dselect as my package manager. It's so easy to manage the text file /etc/apt-sources.list and run dselect, I've never bothered to install aptitude. What could be easier than:
Got those packages? Cool. Dselect can be a little hairy the first time you use it, so don't try to do it in a hurry. Take your time, read the instruction pages and look at the screen to figure things out.
Now we need to put the source where we can play with it without system damage. We will make a directory that belongs to a normal user and unpack the source there. You are still root now.
Now get out of that scary root user and get into your kernel directory and unpack your source:
That's it, you should have a nice new directory that your normal user owns with the kernel source you want in it.