Consistently reproducing handmade items is hard. I've been using 3D modeling (CAD in OnShape) and 3D printing to make cutting / pricking templates, leading to easier, more repeatable leatherwork.

quick rundown:
1. Model the "thing". This is super simple -- a "box bag" would just be a rectangular prism, no fillets or anything.

Use the sheet metal tools to generate a sheet metal enclosure.

For seams, use flanges and scoot faces around

finally, sketch pricking locations & make holes in the sheet metal structure

export the unfolded model to DXG, then import THAT into a new part studio / new sketch

extrude to the thickness of your template. Print that

A simple coin purse might be a cylinder (spoiler alert -- polygons are easier, try an octagon) You can select bends and "faces to exclude." For a 2-part coin purse, you'll make two sheetmetal parts, one per leather cut. The "sides" might be one, the bottom another.

For a box bag, it may be helpful to exclude the face with the zipper. Fold the sheetmetal around the rest of the bag, mark the seams. To represent the zipper, use a flange on the remaining edge(s) to bring the template up to the cut for the zipper. In real life this would be the outside edges of your template, and the sheetmetal model will unfold similarly.