In my scenery book I devote a good deal of space to planning your scenery. This is an important first step if you have a large or complicated track plan.
With some track plans it's very hard to visualize if there's enough clearance between the tracks, especially when one track is above another, or how steep or flat the scenery has to be in another part of the plan, or where you need a rock face or retaining wall.
This is where a 3-D model of the trackplan comes in handy. The scale of the 3-D model is not important. It has to be large enough so you can build the grades to scale. I like one-inch equals one-foot scale. This means drawing a track plan on a grid that's 1"=1' scale.
I make several copies of the plan and glue these copies to cardboard or poster board. I cut out the track outline with a sharp knife, leaving about 1/8" on either side of the line. You'll need several copies of some areas on the plan especially where one track crosses over another.
Square pieces of scrap wood make the risers. These are glued under the cut-out track plan. It's important to estimate the grades so you can determine if there's clearance when one track passes over another.
I like to add scenery using Sculptamold, bit of colored texture, and small buildings made from blocks of wood. It brings the planning model to life.