I am not the final answer here, but I would leave the foot down. In my opinion, picking the foot up and down would take away from the smoothness of a single movement, and add more interuption to the qi flow. It can feel disruptive to lose that balance, but it is only one thing to get a handle on. Move as slowly as possible with the breath and focus on where your center of balance is. Find it when you are shifting your weight to the one foot you are going to stand on. Keep that center balance as you move your other foot and hands up. Your balance will improve with time. One benefit of this exercise is that it improves your balance. You have to start somewhere. Be careful not to throw your center off before you even pick up your foot. Settle your weight to the foot you are going to stand on,as you step, center that point,be aware of your crown point lining up with that center line, ground into the earth, bring your other foot to that center then raise it up. Just in making the initial step and shifting your weight, you establish that centerline balance and alignment before you begin to raise your other foot and your hands.