The biggest problem that results in a poor job, is lack of understanding of the golden mean. If you can't draw a flowing design correctly with no flat spots, don't try to put it on a gun. When I was being taught engraving, I was put to work drawing designs on paper for a month before I was permitted to put a graver to metal. The actual execution can be a bit crude, as long as the artistic expression is done correctly.
You don't need to practice on a gun stock. Make cribbage boards, walking sticks, any piece of wood you can get your hands on, you can practice your drawing and carving skills.