Plant one foot in a turned-out position. You can decide whether you want to start the slide into a split with your right or left foot. The foot that remains stationary should be turned out for stability.
Start sliding slowly with your front foot. Moving slowly is best in the interest of avoiding injury. If at any point you feel a muscle start to pull, especially in the groin area, stop the slide immediately.
Continue the slide until your crotch rests against the floor. Your back knee may be slightly bent, in the interest of avoiding injury. Raise your arms over your head into fifth position, if you like, for a flourish.
Get a running start. Leaping into a split position, as does any elevated grand allegro movement, requires momentum. You'll need at least two to three steps to launch yourself into position. If you are practicing on your own, you can just take two or three running steps from any position. If you are in class, you will likely be asked to perform a moving step such as a pas de bourree or glissade in preparation.
Do a developpe with your front leg as you push off from the floor with your rear leg. A developpe is an unfolding of the leg. It might be otherwise termed kicking your leg forward from a bent-knee position.
Widen the legs into a split position while you are in the air. This takes practice and strength, but makes for a remarkable sight. Widening your leg position while you are aloft gives the appearance of hanging in the air for a moment.
Descend into a deep plie, or knee bend, as you land on your front foot. Slide the rear foot in behind it and bend that knee as well. Landing on a bent knee is important to prevent injury.