Ron Micjan's page is excellent, so is Kim Meinecke's (sp?). Kim placed the O2 addition before the scrubber to aid mixing the breathing gas, which I like better, even though it means introducing a dry and cold gas into the scrubber. It's the little details that make it hard to build a good rig. Also, there are several people who converted other SCRs (Ray, Azi, EDO-04) and O2 rigs (Dave Sutton comes to mind, do NOT miss his page - any of it), all of whom had slighty different ideas. Most of them can be found on JW Bech's Rbreather Site.
Whatever you decide to do, make sure you think it through. Then debate it with someone else to catch what you didn't think of. You should know how each and every modification to your rig will influence it and how it dives.
The how it dives part is important, too. Plenty of diffrences between a cmfSCR and a KISS valved CCR. Seriously consider instruction on a KISS CCR to learn all there is about the valve and how to dive it.