I have constructed the baseboard out of 3mm plywood and two 20mm by 40mm 2.4m lengths of pine. A frame was made out of the two lenths of pine measuring 2.0m by 0.4m. The 3mm plywood was then cut to size and glued onto the frame with No More Nails.
I would have used thicker ply for the baseboard but I chose to use thin 3mm thick because it is much cheaper and it doesn't need to be strong as it will support nothing of great weight. If you are not sure of weight limit for plywood, invest in woodworking tools that can inform you of the correct usage.
To make the frame I cut and joined the pine together using butt joints. Butt joints are the simplest of joints but they are not very strong as they have little contact surface area in comparison to other joints. Using screws as well as glue on these joints add to the strength.
Above is the design of the frame of pine I used to support the top of the baseboard. It's best to have no more than 1.5 ft of the top of your baseboard unsupported.
I then spread some no more nails onto the top side of the frame. Next I positioned the 3mm ply ontop and used weights to hold the ply down onto the frame. I then left it over night to dry.
After I had the baseboard sorted I started to put the supports in for the shape of the relief (hills), for this I used formers which are peices of wood which are shaped to the profile of the landscape. The formers are made from scrap pine, they don't need to look good as they will be covered one the layout is finished. The formers were then glued to the baseboard with no more nails.
The photo above shows the baseboard and landscape formers for the valley and cuttings. In the distance the station end of the layout can just be seen.
The track level is rased above the baseboard at the station end using scrap wood and 3mm ply. The wood was cut into blacks with the ply cut to shape, and placed on top. Gaps were left to include bridges.