The following video presentations outline some reasons for using a grid system, concepts you need to know before designing one and then walks you through the process of creating your own HTML/CSS Simple Grid System.
Part 1: Understanding Grids
http://www.youtube.com/watch?v=76nYNZH4bMY
download: SimpleGridSystem1.mov
Part 2: Web Grid Design Basic Ideas
http://www.youtube.com/watch?v=oP-y15-wpVQ
download: SimpleGridSystem2.mov
Part 3: Implementing a Simple Grid System
http://www.youtube.com/watch?v=EjKvTTVMgF4
download: SimpleGridSystem3.mov
Here is a pdf file of the presentation: Simple Grid System Slides (PDF)