I’ll try my best to describe the procedure that I did when I was doing floor coverings, which amounts to tracing the contour.
First you will need a couple of saw horses or something to hold the plywood up next to the boat at the precise elevation and angle to the floor of the boat. Since the lower in the boat you go the narrower your floor will be. And make sure that the plywood is parallel to the boat. Once you have everything where you think it should be grab a stick or a 1X2 something ridged so you can trace the contour of the boats hull onto the plywood. When you cut it out you will want to use a jig saw and cut it at an angle less than 90 degrees, probably close to 60 degrees so the bottom edge of the plywood does not touch the side of the boat before the top edge is close enough.
If you go to an appliance store you might be able to get a piece of card board to practice on…