Try the guitar in the places where you really use it. Stand and play a few familiar songs. Reach for the higher frets. If you sing while playing, notice whether the guitar sits naturally when you take a full breath. Then sit down with it. The guitar does not need to land in exactly the same position in both situations, but it should remain easy to control without the strap pulling, bunching, or putting a hard edge where you feel it.
Pay attention to the details that emerge after a few minutes, not only the first try-on. Does the outside shoulder begin to feel a narrow pressure point? Does the body drift farther from you than you want? Does the neck-end attachment twist when you sit? Those observations are more useful than a general sense that a guitar feels heavy. They help determine whether a different width, a padded section, an adjustable fitting, or a new playing height will serve you better.
The existing acoustic guitar strap guide walks through attachment, fit, and comfort in more detail. When you are ready to make the strap personal, bring a photo of the guitar, notes about your normal playing position, and any color or leatherwork direction that keeps drawing your eye. You do not need a finished design to begin.