Get a bunch of heavy brown paper to use in testing concepts. Then experiment with a front piece. I like a slightly deep D shape just big enough for me to comfortably get my hand inside. Draw onto the bag and fold corner to corner before cutting. Cut it folded so you get symmetry.
Once you have a pleasing size and shape, transfer the body shape to bottom of the rear piece, but leave a flat to serve as a cover. Then add a connecting strip to join these. I like to flare the bottom a bit (by using a wider connecting strip) to create a bit more room at the bottom.
You should also consider a pocket inside for special items and/or maybe a pocket on the back for special items.
Some people like a patch knife sheath sewn to the back of the bag. I prefer to put my knife on my belt.
Common errors that I have seen and/or done:
Bigger is not better for a shooting bag.
People often make the strap too long so that it hangs and dangles in an inconvenient manner. Adjustable is a good idea for your first bag.
After you have done a few, you will learn your preferred length.
Avoid knife sheaths on straps since these tend to deposit the knife onto the ground. Got one set up that way and will not do that again. You might come up with some great securing strap that answers this problem, but I see none that look correct for my chosen times/places.
Good luck with your bag project!
CS
