My advice is to always prepare your practices on a paper. It can be written down on a napkin or printed out professionally on your practice template. The form does not really matter in my mind.
You will see that players will be more engaged. For me this small action shows respect to your players and that you took the time to prepare something. A lot of times I spend more time on the practice plan than the practice itself.
Your goal is to create the most efficient practice plan, that is fun for your players, teaches them something in depth and gives them as much action and reps as possible. But for me the plan should also keep open space for the opportunity to experiment and try new things/drills while still being efficient and have a practice where we work on specific qualities. My plan is always too long. It serves as a guideline for how my practice should go but I always go off script a little. I allow myself this.
If you come up with a new load at the spot, or it doesn’t work out the way you thought it would then I start to improvise.