To enter this posture, your hips must feel nimble, open, and warm, as the pliability of your hips will largely dictate your ability to get your legs onto or near your shoulders and stay there. For this reason, it's best to do this pose after you have performed several sun salutations. In a sequence, I often place this pose after a squat or prasarita (expanded leg intense stretch or "straddle").
From prasarita (a forward bend with legs wide apart), walk your hands along the floor until then are planted flat, beyond your heels; your fingertips pointing forward. Remember, your hands are shoulder width apart, just like any other arm balance.
Bend your knees, and sit back onto your upper arms. Then, lift your feet off the ground. If you find that your arms don't feel long enough and you can't properly flatten your palms, try placing a yoga block under each hand. This will give you more levity.
Unlike many arm balances, the tendency to fall will lead you backward, as opposed to forward. If you're scared, consider that the floor is not that far away, and your tuckus can probably handle a short topple backwards. Eventually, you will feel weightless in this posture and strong, knowing that you can support your weight with your own two hands.