So much to say, but I’ll keep it short . . .I mark the seams by selecting the edges, then ctrl E > mark seams, then U and unwrap, Blender will unwrap it to a 2D image. Experiment with the best way to mark your seams. You can clear the seams, and try again if it didn’t unwrap to your satisfaction. Make sure you set the bleed to something higher than default .2. The bleed is in texture paint, options. Otherwise, when you paint, or clone on your mesh, you’ll get edge flashing. There are many other ways I’ve done this. including color coding the model and exporting to PS, or Substance Painter, or cloning from an image, painting on the mesh, or on the UVImage. among others. I can’t decide which is the best. Sometimes I use the “Unwrap image from view” option, as it sometimes gives me a better result. In the UVimage editer, in edit mode, you can stretch the vertices to get rid of the stretched look on your mesh. You kinda want to make sure the islands are fairly close to being the same size. Maybe someone can explain better.