It is hard to find an easy to understand set of instructions for creating a custom debian installer the real way. And when I say the 'real way' I mean using the same tool that the debian folks use to create their ISOs.
Read it here
Before we get to the nuts and bolts, here is the high level of what we are going to do. We are creating a bootable debian-installer ISO. To do this we use a tool called debian-cd, which is basically a build file and lots of perl scripts that automate everything. debian-cd requires that we have a local debian repository, and that is where most of the work is. Then you configure debian-cd to point to your local repository, and you tell it what you want it to make, there are tons of options here, but I am going to give instructions for the i386 netinstaller CD. Once it is configured you just run a few make tasks and you are done. Now without further blabbing I'll cut to the chase.