After you've sewed all your blocks and pressed them meticulously, it's time to trim them to size so you can easily assemble your quilt. The best way is with a plastic square ruler; I'm assuming your ruler is larger than your block (although the easiest of all is to trim your blocks to exactly the same size as your plastic square; I have learned to love 6-inch blocks that finish to 5 1/2 inches square).
Hold your ruler so the zero measurement on each side falls at the top right corner (lefties stage up at the top left). The ruler will have a diagonal line on it heading down and out from that corner. Follow along that line to the size you want your block to be, and mark the spot with a grease pencil or a bit of blue painter's tape.
If you want your blocks to be rectangular instead of square, put your painter's tape or grease pencil mark at the right place on your plastic ruler and then use the same method to align the bottom left corner of your block with the mark.
You will notice that my original block was larger than the finished size, leaving a bit to cut off. The quilt police would tell you that if you want an eight-inch block, you should start with a bunch of eight-inch-long strips and carefully sew them together. I say you should start with 8 1/4" strips, or maybe even 8 1/2" if you're going to be doing tricky piecing, and then trim the block to size after it's all sewed and pressed.
No matter how carefully you sew, things happen and your exactly eight-inch strips may get staggered out of position and leave you with a block that's a bit wonky or too small. That is frustrating. Better to start with a bit of extra fabric, sew with less fussiness (and more speed) and make things perfect at the end.