Instructions
Find an applicable game online that allows you to design a roller coaster for free. You can find free games for roller-coaster building at Roblox, Discovery Kids and Learner.org.
Load up the game and read over the creation rules. Some games allow you to build the coaster piece by piece, while others provide you large sections of track that you can add in like puzzle pieces to create your coaster of choice. Once you have the rules set out, lay all of your track and make sure that you have a completed coaster. Do not leave gaps where the car will fall off once the simulator starts, or create other, similar problems.
Run the simulator. Once you have a fully completed coaster you can give it a test run. You can watch the coaster run through the track you created, and if all goes well come back safely to the start.