As you make your blocks, you should try to sew a consistant 1/4 inch seam throughout. If you have succeeded, than your block should come out the size indicated on the pattern. If it is over a little you can sometimes use a large rotary square ruler and rotary cutter to trim off the extra. I said sometimes because you have to careful that all your points are still going to be 1/4 inch from the edge (seam allowance)and that the design is still centered. If all your blocks are square than your quilt top should be square.
If your top is already sewn together and is out of square than it's a different process. If you have large borders than you can trim a little off without it being too noticeable. I like to wait until my quilt is quilted before trimming the edge. This is my preference, many do it differently and you'll have to discover what you like to do.