Line up the fabric's sides, where it tends to be straight and not where the store cut the fabric, which is not as reliably even. Then place your block on top and mark where you need to cut. You'll get a long strip. Just lay your paper pattern on top of the strip again and mark off the side seams. This gives me pretty straight and even blocks.
After you've cut all the blocks you'll need, it's time to figure out where to sew it all together. Fold your paper pattern where the 1/2-inch mark is and trace it with either a pencil or pen. You can buy special fabric pens that fade in the wash. I've used black ink pens on light colors and since it's on the inside, it doesn't show anyway.