I'd do it like this:
1. Pick a hull and trim it to the length you want plus the crimp.
2. Choose the shot weight you want to shoot, measure the volume of that.
3. Weigh that same
volume of powder and subtract 10%. That's your powder charge.
4. Prime the hull and pour your powder in.
5. Seat a 1/8" over powder card firmly on the powder.
6. Pour in your volume of shot and measure the length remaining above the shot to the case mouth. Subtract 1/4" for crimp.
7. Your cushion wad will be that length from the crimp to the shot.
8. pour out the shot and seat that length of cushion wad and pour the shot back in.
9. Put an over shot card over the shot and spin down a nice roll crimp.
There, you've made your first shell. The next ones will be simple repedition. Go shoot some paper and see how they pattern. Then, get an old bed sheet and spray paint a SASS size target on it. Hang it up and step back to the distance used by your club for most shotgun stage targets. Mount, point and shoot the pattern. Don't aim, just point and shoot. Do this over and over again till a ragged hole shows up. This is where your shotgun is hitting. Allow for the difference between your actual hits and the target center when you point and shoot. Soon enough you will be a skilled shotgunner. Never tell this secret of how you can just point and shoot your scattergun and hit so well.
DD-MDA