Koosh koosh is the easiest way to wheel walk backwards. Place the first foot on the frame, overlapping onto the front of the tire (locking it in place). Then place the second foot on the tire behind the frame.
Start by a wall to get the feel. Now attempt to move backwards. As you lift your front foot up and off the tire, push back with the foot behind. Then lock the tire in place again with your front foot and reposition your back foot to take another step. Try to make your pushes with the back foot as big as you can. Soon you will be able to leave the wall behind. Learn to get into koosh koosh by idling and then locking the tire when you have completed a forward idle. Then you will be ready to move backwards. To get out of koosh koosh, try wheel walking forward.
Fast fact: this trick is named for the funny sound it makes on the tire. :)