I do a lot of cut large and trim to fit. In this case I would cut the square correctly and make the side triangles a little large, Just sort of center the oversized triangles. You don't need a whole lot of extra, just enough to make sure your sides and bottom are full, so if you care cutting the sides as a square, make it 1/2" larger (so 1/4 for each of the triangles).
For trimming, the top part is easy. Just use a corner/square ruler for the top -- you should not be trimming off any of the square, just the triangle sides. For the long cut on the bottom, I would mark my ruler using tape. Again, using the square as correct I would position the ruler and just do a fast cut along the bottom.