You must be efficient with both tricks in a combination to missleg.  Precise balance is necessary for the first landing, and efficient technique in the second trick are the only real pre-requisites to perform this transition.  Its often possible to simply muscle, or power through this transition with poor technique, but this often leaves the second trick somewhat less controlled and often difficult to combo out of well.  Working up into a missleg transition can be difficult at times, but there is a way to work toward the muscle memory and general feel of performing one.  First, decide which trick you wish to missleg into, and another trick that will setup a reversal transition into first.  Next perform the combo as a reversal until it is efficient.  Once comfortable, begin to utilize the second landing leg less and less.  The key is to rely entirely on the base leg for balance and jumping, while using the second leg mainly for lift, without pushing off of the ground very hard.  Once this becomes comfortable, it should only be a matter of not touching the second leg to the ground as you transition into the second trick.