Whatever the vehicle is, you need to grab the handle bars and turn them to steer, while keeping balanced.
The superbikes are the hardest by far, because of the speed. You need to do very small movements on these, the chest and lumbar have had their range reduced so that you can do smaller movements with them, so use those joints as much as you can to help.
The dirtbikes are more about keeping upright while pointing the front wheel forwards all the time.
The quads are the easiest to steer, and are very easy to control. It's best to grip the body of the bike with your legs, contracting your glutes, hips and knees. This gives you sharper steering. Again, chest and lumbar for the fine steering, but you should use your whole upper body if you need tighter turns.
Check out this replay from me and Imapirate. I think this was only his third try on the mod, so you can see it's not so hard.