Design the layout of your roller coaster. You might want to simulate your favorite coaster, in which case you might need to go take photographs of it from different angles. You can also find pictures of many roller coasters on the Internet. Use graph paper and a pencil to sketch out a model, to scale, so you know how large your base will need to be. If you choose a model train scale, such as the HO size, you can buy track instead of making it.
Paint your plywood in the color you want. The size will depend on the model you're building. To cut a plywood sheet to make a smaller piece to support your model, use a handsaw or circular saw. If you're recreating a roller coaster from an amusement park, you might choose green for the grass under and around the coaster, or you might choose gray if the surroundings are mostly cement. Then, mark the line of your track on the plywood with the pencil once the paint has dried.
Erect your model on the base. This will involve setting up your balsa wood supports, consisting of poles coming up from the plywood base, and then applying the track to it. If you're simulating a metal track, such as the Shock Wave at Six Flags over Texas or Superman at Six Flags Fiesta Texas, you'll want thicker balsa sticks, since there aren't very many support poles. Choose sticks that are at least 1/2 inch in diameter, glue them into place onto the base and then apply the track on top.
Use narrower balsa wood sticks for recreations of wooden roller coasters, as their frames are intricate and have thousands and thousands of beams. While your model doesn't have to have all of those beams, you'll want to use narrower sticks, because you'll have so many pieces. These balsa sticks will show the intersections of triangles that form the trusses inside the supports. You'll run vertical sticks from the base up to the track, as well as diagonal sticks up from the base, at a 45-degree angle, until they hit the track. In a real wooden roller coaster, the supports bolt together at every junction. Because this is just a model, you can use wood glue or twine to connect the 45-degree and vertical supports where they pass one another.