I make my benchwork from cheap lumber commonly sold at building and home improvement stores in the USA. They are called 2x4 studs (actually 1½" by 3½") and typically come in 8' lengths. Don't buy too many at a time, they can warp and twist if you don't use them for too long.
I mount these studs to the walls at floor level as well as at the desired height (lowest track) of the layout surface. I then build a framework using joist hangers and drywall screws in a horizontal plane. Support is then provided by studs at 45° from the stud at the floor level.
For benchwork that does not run along a wall, such as an area that protrudes into the middle of the room, you cannot escape having some legs. For those I create adjustable legs from, studs.
Once the framework is in place and supported with the diagonal braces, drill a bunch of holes in the framework for threading wires. When you have done that, drill a whole bunch more, because you will need more than you think.
Thread the main bus wires. You will need the following bus wires:
- Common ground (Heavy guage, I use brown)
- Track power digital signal (Heavy guage. I use red)
- Accessory power (I like to use 12 Volts DC, Heavy guage, I use white)
- Multi-strand cable for track detection feedback wires.
In addition you will later be installing wires for signals and various accessories.
Bring track power close to where you expect the tracks to be. If you expect to have multiple power districts with boosters, decide now where those districts will be and provide separate track power to those areas. Don'f forget to run wires from your power supply to where you control area will be too.
If the upper surface is going to have a slope, you can use ¼" threaded rods to hold a sheet of plywood at the appropriate height. I drill a hole into the framework, then drive the threaded rod into the hole. To drive the rod, lock two nuts together at the top and drive it in using a drill with a socket that matches the size of the nuts. Once in, unlock the two nuts and move them down the rod to whatever height is needed. On top of that I thread a strip of wood or metal to provide support for the plywood roadbed.
Once the supports are in place, you can add the plywood that will support the track.
Adjust the heights, then hold the plywood down by adding a washer and another nut.
I suggest using ⅝" plywood.
In places where there you need to support multiple layers, rods can be added as needed.
For layers that are horizontal it is also easy to make supports by cutting short lengths of wood and screwing them to the sides of the frame. This allows the plywood to be securely fastened down from above.
If track is going to be hidden by upper layers of benchwork, lay and wire up the track (including train detection) that will be hidden before covering it. Test it thoroughly.
In order to prevent the wires from an upper level from fouling trains below, I partially screwed a series of 1/4" screws into the underside of the upper layer before installing the board. Then when I lay track above, I drill through and thread the wire through the hole and then lead it to the nearest screw, wrap it around twice and pass the end between itself and the board and then lead it down to below the lowest level where I tap into the bus lines...
Add LED lighting on the underside of layers that are above hidden tracks.
Plan on hanging curtains on the front edge of the layout so that one cannot see through the landscape.
Once you have the top benchwork in place, you can at last start laying visible track!