Upholstery and Fabrics - Help with Upholestry

I am upholestrying my husband's pantoon boat seats. We had to rebuild them from scratch. My problem is I can't remember how to cut the corners so there won't be folds in the vinyl. This is my first attempt in this big of a job. I also just want to make sure I am doing it right.

Are you just pulling the vinyl over the seat and stapling or are you sewing cushion covers?
If you're stapling, cut the corners square, then you want to staple a couple centers first to hold the fabric taut, then kind of pull it tight and under at each corner (one at a time) and smooth it and staple as you go. After it's stapled, you can cut off the excess. Hope you understand as it's a little hard to explain. You could also form small, accordian like pleats, going around the corner, which looks nice.
As long as you cut the vinyl bigger than what you need, you should be OK and you can remove the staples if it doesn't look nice and try again. After it's all stapled and looks good, then cut off the excess fabric. It's when you cut too small, or to shape, that you run into problems.

I understand what you are saying. I know this might sound stupid but what do you mean by cutting the corners square can you explain?