Tandy has/had a small book on holster making, YEEHAW! In it there is a chapter, with kindy-garden type illustrations on making patterns using THE STRAP!
Not the one Daddy had in the barn, but something like the reins used with the plow-horses. First get some pattern paper. (Target material, old file folders or large document envelopes) Draw a line down the middle or fold a line. Hold the pistol, sight line down on the line and carefully lay it down on one side and trace the outline. Then draw the leather-line about 3/4" outside that. Fold the pattern paper and copy it for the other side. An easy way to do this is with scissors, fold & clip, just like making crafts in kindy-garden. Pretty rough and ready, so based on the carpenter's rule "Measure twice - cut once", now get out THE STRAP!
A strip of leather of the same thickness as your holster material. If you are lining the holster, the strap has to take account of that. Pinch it into a loop and take the circumference of the pistol at critical points, including the 3/4s of an inch you are holding in the pinch. (Actual plus 1 1/2") use these measurements to lay out a pattern on heavy paper. OR use them to alter an existing measurement. Sometimes I make several tries before getting a pattern I think I can use. If you are new to this, make the first holster simply, without a lot of time and resourses. It might become your loaner or spare - hopefully not returned to the scrap pile!
Once you have a successful pattern - keep it on file, properly labelled. It can be altered for such things as barrel length, target sights and style of pistol. E.g; if you want to make a holster for the Old Vaquero but only have a SAA pattern, "ease" the pattern by 1/8' in all dimensions. Even a New Vaquero is just slightly larger than a SAA clone (personal experience - Vaquero is very tight in a holster made snuggly for the SAA.)