Lots of good info here.
I'll toss in what i learned in building a track this summer.
having the right equipment is key. A d6 bulldozer or equivalent would be ideal, along with a front end loader and possibly a dump truck depending how far and readily available the dirt is. We were lucky in that the property had a cone of 1000+ yards of screened top soil that we used to build some of the features and a good portion of the track was built on a massive pile of dirt (hundreds of thousands of yards of material if not millions) that had been scraped from the surrounding area when the property was going to become a golf course before the project fell through. So luckily we didn't have to transport much dirt, but if you do you want to be able to do it in large quantities.
be ready to burn lots of diesel and lots of time. We were typically running a Komatsu 200 excavator or a komatsu 138 and a kubota asv90 skid. I probably spent 150 or so hours working on it along with another guy who spent around the same amount of time, the track is about a mile long on 15ish acres id guess.. Unfortunately we didn't have a bulldozer which would have saved a massive amount of time. the excavators were good, but it took way more time to grade and cut in the track then it would of with a bulldozer.
come up with your layout and use stakes to mark obstacles and corners, it can be hard to gauge distances for jumps/features and radius of corners when you are operating.
get the layout done and ride the layout some before building a majority of the features. this will let you get a feel for the speed of the track and will help you determine what features might work where and if you need to change anything layout wise.
don't make your jump faces too steep. I had the best luck building jump faces as a ramp, with a smooth transition at the bottom, once through the transition the face of the jump maintains a consistent slope, think of a triangle.
make things as low consequence as possible at first so you aren't intimidated to hit it and if you mess up it won't be to bad. as you get better at building you can make features of higher consequence cause you will know what works and what doesn't.
if you have good top soil to use try mixing in some bark mulch or sand with it to help keep it broken up. most top soil has quite a bit of clay in it and it will become rock hard with machines driving over it all the time.
having a power rake attachment or something to till the soil when you are done would be a massive help in maintaining the track. As someone said earlier it's the outgoing grooming and maintenance that is a real doozy
Have fun!