Gents you may find useful basic information on DIY lapidary tools at Kreigh's Homemade Lapidary Equipment. About half way down the page the useful sections start; Make Your Own Rock Saw, reciprocating wire saw, Some Thoughts On Horizontal Laps.

A google search for diy rock saw will lead to inspiration. For really big rocks search for drop saws, diameter up to 60". For moderate sized rocks a slab saw will do very well, blade diameter 10" - 24". If you are going to flatten a lot of stones a vibrating lap would allow you to set it and walk away for hours at a time.

Keep having fun with your rocks.
Jonathan