Hi Joe, what you're asking about is often referred to as subroadbed. HO-scalers can get away with using Homasote or lauan plywood. But with typical spacing of risers, there's no way those materials would support the weight of an O gauge Big Boy! I agree that there just aren't enough illustrated step-by-step guides for O scale.
I bought all of the popular Kalmbach books about benchwork, and watched videos by Joe Fugate, Model Railroad Academy, etc. Personally I think spline roadbed is labor-intensive, and overkill for what you're trying to do. It might even be more difficult than 3/4" plywood to bend in the vertical plane for making grades, etc.
Plotting your track plan from SCARM, RR-Track, etc., in 1:1 is probably a worthwhile investment. A commercial printer like Kinkos, Ridgeway, or even Office Max should be able to do that for a fee. Next, I would cut out long sections of track from the paper plot, and glue them to a 4x8 sheet of high-quality 1/2" plywood, arranged so as to minimize waste. Then, fire up the jigsaw! Cut the sub-roadbed wider than the ties. For sections of double mainline, yard, etc., you might use a single, wide piece of subroadbed to support the whole thing. Be patient with yourself and start with plenty of cash, because mistakes will be made and material will be wasted! If you want to make an omelet, you have to break eggs!
Yes, there will be splices! If they are carefully assembled and well-supported, there should be no impact to operation of the trains. I would have bubble levels of different lengths on hand-- everything from 4" long (i.e., the wheelbase of a steam loco) to perhaps 30" long. The idea is that you don't want any bumps, humps, or sudden changes in slope. Having a partner to adjust the height of the risers with clamps will help get everything nice and smooth before you screw things together permanently. Also, an ability to make beveled cuts could be helpful, because I think sometimes the "tops" of the risers would have to be cut at a slight angle if they are supporting the sub-roadbed on a grade.
I'm right there with you in trying to learn this stuff, and had all the same questions you do, so I'll be following this thread! For those of you with firsthand experience, please don't hesitate to correct any misinformation!
