I'm not claiming to be any sort of track building expert, however, I do know that you haven't provided enough details for people to help you with figuring out this project.

You need to determine the length and the width of your usable area. Once you know that, you can easily draw out sketches.

Jumps, ramps, bumps could be as high as you want them. Keep in mind you will be displacing a lot of dirt, and you shouldn't be taking it from the track site. (Any deep gullys will fill up with water first rain) It's probably best to start your jumps smaller, and build them up as you need to.

Any elevations should be reinforced at the edges with wood plank or board, keeping your dirt where you want it. These elevations must be tamped to compress the elevation and harden the track.

Just an idea:
Most tracks I've seen look very manicured throughout, very compacted dirt, and no mud/loose dirt. What would be neat is having an area on the couse that was a loose sandy dirt, that you raked like a sandtrap on a golf course. This adds another element to the track, forcing the drivers to use different skills.