The first time you're not going to have as much control as you're used to having. It will be tough to do this, but now you will begin your mental progression of learning how to jump your dirtbike while not having as much control over it as you would like.
The first thing to do when going for a one hander is to become accustomed with no feeling of control over the bike. It will seem weird not having your hand on the bike, but thats completely normal. When you leave the jump make sure that your bike is level with the ground. If you take your hand off and the bike is pitched awkwardly, you could lose control or come off the bike in a way not desired. The best way to get used to this feeling is to open up your hand when you are in the air. Don't move it away from the handlebar, just release the pressure from your fingers around the grip. After you are accustomed to this movement , you can start moving your hand a little ways away from the handlebar. Each time, little by little, start moving your hand a little bit farther from the handlebars. When get to the point where taking your hand a way from the bars is no problem, start moving your hand behind your back and stretch it out to the point that its pointing towards your fender. When you can safely do this, you have got the one hander down.