The plywood sub-roadbed is the same that I used for the layout helix and the staging deck sub-roadbed, it is 5 ply, 15/32" (12mm).
The first step was to align the curved section of track into the first curve of the return loop.
Once this was installed and the transition pice installed I was able to move onto adding the remaining sections of the return loop plywood.
The joins between section were secured using perforated sticking plate and stiching screws. This was the same method I used on the helix.
Once I had the two full sections installed I moved onto installing the straight transition section from the last return loop curve back onto the plywood staging deck, this required a hole to be cut through the backboard.
Tomorrow I'll finish off the remaining braces that need to be installed, and secure the return-loop plywood curves down to the braces and install the track.