I made one in a overgrown field. First took out the riding lawn mower and tried to make a course. Had to do revisions once on the bike as the lawn mower didnt go fast enough to get the feel of a turn. :) I tried to route it over any elevation change as it sometimes gives you that low/far jump out of it if going fast enough. Also it was an old farm field so there was some sections that still had the plow marks making a mini whoops sections. Other than that you need to add dirt for jumps if you want it like an mx track.
Riding, and laying out, grass tracks can be a ton of fun. Especially, when you have someone else of equal riding ability to race on it against.
Riding them by yourself is a good way to learn advanced turning techniques for non rutted, non bermed turns.
Get some cones and buckets and put them in places to mark the turns. These work well because they are easily moved when you want to change the nature or severity of the turns.
One you have your track worn in, you may want to use more permanent markings like stakes, ribbon or hay bales or whatever else you have. If you have a lot of trees, you can rake leaves and put them in a 55 gal plastic trash bag to make some soft course markers.