Instructions
Find an unused spot of land to build your ride and dig down to the level you want your ride to start at, digging out just a few spaces. Place your ride's station platform in the hole, currently above ground.
Plant the next station platform underground, directly next to the above-ground station platform. Remove the first station platform and raise the land back to your desired height.
Build the ride as you would any other, only underground. When finished, connect the end of the ride to the other end of the station platform.