Press first. Each and every seam, then turn it over and press again. I use my dining table and large cutting mat. Start at the upper right corner. If you have a large square ruler, square the corner and then use your 24" ruler to trim the edges, if needed. Invariably, if each block is square you will have very few problems squaring the finished top. Also the binding can hide a multiple "sins". My are usually in the corners.