Here are the steps to building a ride underground

Build some regular coaster track, not station track

Build a tunnel and make the track go underground. RCT will automatically switch to underground view, like with regular coaster tracks and paths

Build the station

Delete the track behind the station, so nothing but the station is left

Build the rest of the coaster. Regular paths and tunnels can be used to link up the entrance. Make good use of the height markers to help in linking the track back up and building the paths. Exit platforms are about 3 units high, while entrances are about 5 units high. All coaster track is 2 track units high, except in the case of inverteds and suspendeds, which are 3 units high.

To build flat rides underground, the first step is to make a hole to put the ride in. The next step is to either cover it with paths or a pyramid. A pyramid only has to have its outer edges on flat land, so things can be built under the pyramid. A trainer can also be used to raise the land to above the ride