Parking, or making any kind of turn with a tandem boat trailer, is a complicated process. Although moving a single axle trailer is not too difficult, you may find that a tandem axle gives you all kinds of problems. The main difficulty is locating the pivot point of your trailer, as you now have two different axles. You can no longer work along the rotation axis, but need to locate a point somewhere near the center of the two axles to find the right movement.
Step 1 - Prepare to Turn
Start by setting up your trailer at a 90 degree angle to the parking space. You need enough space to turn the tandem boat trailer, and ideally the length of the trailer should make up both sides of an imaginary box, into which the trailer will need to be turned.
Step 2 - Parking
Begin the turn by moving your trailer handle to between 15 and 20 degrees. Power up the trailer, and move slowly around. Have a friend monitor your position, at least for the first few turns. Once the trailer is 50 percent around the corner, you should then be able to reverse the process by moving your handle to the opposite side of the wheel and turning the trailer. Repeat until the tandem boat trailer is parked.