If you mean starting from a straddle stand, then put your feet on an elevated mat and your hands on the floor. Stack the mats until you are successful. Every time you get three in a row, lower the mat until you are doing it on the floor.
If you are trying to do it from a sit, start with your legs resting on your elbows. This helps you to get the skill without having to hold the press and starts you up higher than a press so that you don't have as far to push. Once you are successful there, try it on an elevated surface with your legs dangling to the side -- gives you space to get the mechanics without having to hold the press to start. After you are successful there, then try it on the floor. If you get to the floor and are still having some trouble, try rolling back with your legs in a straddle, then rolling forward and pushing into it. The momentum helps you to be successful and just doing it helps to build the muscle strength that you need.