Here is a cut-away close up of the spline roadbed I use. Notice I use 5 splines under the track, then I take one additional spline and rip it at a 45° angle and glue it on each side of the roadbed to form the ballast shoulders. The lip underneath the ballast shoulder provides an excellent place to anchor scenery.
The splines are cut from 1/4" masonite hardboard, and are 3/4" wide. I glue 5 of them together on edge with yellow carpenter's glue, clamp them every few inches with clamps, and let them set up overnight. Then I remove the clamps and run a sureform plane over the top of the spline a few times to remove any hardened glue bumps.
I cover all the details of doing this spline roadbed in the March 2000 issue of Model Railroader magazine.
I also demo constructing EasySpline on volume 2 of the Siskiyou Line video series, and I show how to lay flex track on the spline using latex caulk.