Learning to ride in a half pipe means learning new ways to do everything that you have done before. The dual transitions change all the angles that you are used to when snowboarding and thus change how you perform all of your tricks. This is true even for the most basic of all tricks, slide turns.
The first step to learning to ride in a half pipe is to get used to riding Fakie and the transitions. At first you may prefer to start riding a half pipe from the bottom of the ramp rather than dropping in from the top. The advantage to starting from the bottom of the ramp is that the speeds are lower so you have more time to get used to the feel of the ramp. While getting used to the feel of the ramp ride up one side and Fakie back down the same side.
Once you are comfortable riding the transitions of the half pipe you can begin the art of slide turning in the half pipe. A slide turn on the half pipe is different from a normal one, as you have to roll the edge of your snowboard in order to do it. Rolling the edge means you ride up the transition on one edge of your snowboard and back down the transition on the other.
As is true for any turn you must begin with the correct posture. Your knees should be slightly bent and your shoulders pointing in the direction you are travelling with your weight equally centered. As you ride up the transition find the point where you wish to turn and look at it as you approach it.
The process of making your turn is very similar to making a skidded turn. As you approach the point where you wish to turn begin by turning your head and hips to point in the direction that you intend to go. As your hips begin to twist you can start to roll your snowboard. Roll your snowboard so that the appropriate edge at the front of your snowboard only begins to bite in. This step is important as it reduces the radius of your turn, critical on a narrow ramp. As the nose of your snowboard bites in slightly reduce the weight on your rear foot and push your leg out in the direction that you want the snowboard to swing.
Once you complete your turn you will want to change the weighting of your stance. You must roll your snowboard so that your weight is now slightly towards the opposite edge of your snowboard. This action acts as a stabilizing effect to help you get orientated and moving as quickly as possible straight back down the half pipe.
As you can see slide turns and other maneuvers done on half pipes are very similar to maneuvers done on the slopes. While the basics of these maneuvers are the same the key to them is being able to do them quickly and accurately and that only comes from lots of practice.