Lay your 18-inch plywood flat and set four swivel wheels on top of it. Position a wheel in each corner so the brackets are flush with the corners of the plywood. Screw the wheels to the plywood using the screws provided. This is the rear dolly for your snowmobile.
Position two 20-inch boards flat, parallel and 7 inches apart. Lay a 20-inch sheet of plywood on top of the boards so the edges are flush. Screw eight screws through the plywood so four screws enter each 20-inch board. Repeat this step with the other two 20-inch boards and 20-inch plywood. These are the frames to your ski dollies.
Set one of the ski dolly frames flat so the plywood is on top of the boards. Position a swivel wheel inside each corner of the plywood so the edges are flush, just as you did in the first step with the other sheet of plywood. Screw the swivel wheel to the plywood using the screws provided. Repeat this step with the last set of wheels and ski dolly frame.
Lift the back end of the snowmobile a few inches off of the ground and slide the rear dolly completely underneath the track. Lift each ski up and set it between the 20-inch boards on the ski dollies.