Yes. Always make one first, make your adjustments as needed, make another out of leather if your satisfied then you can make your pattern out of plastic if that is what you want to use. I also mark on my patterns little notes like steps in the process of putting that item together, weight of leather used and the size of my holes punched. I find I save on leather due to less mistakes and don't waste that much time trying to remember how I made something in the past.
Also being organized in your work area really helps.