How do I stop improvising and start planning my shoots?

You show up and figure it out on the day, and you can feel the cost of that in the work. Improvising has its own kind of energy, but when it is your only plan, the cracks show: missed shots, wasted time, results that fall short of what you pictured. You already know this, which is why you are asking. Planning does not kill the spark you get from improvising. It protects it. When the logistics are handled ahead of time, your creative energy is free to go where it matters instead of scrambling to cover what you forgot. Preparation is what buys you room to be spontaneous where it counts. So before your next shoot, spend even thirty minutes writing down what you need and the order you will do it in. Just a simple list. You will be amazed how much better the work gets when your mind is not busy remembering. Plan the boring parts, and free yourself for the good ones.