You may want to lightly mark a 1/2" line on the back of your blocks to sew on, just be careful what you use so it comes out in the laundry and does not bleed or show through the front of the block.. Make sure you line up the "intersetions" and pin as needed.
Most patterns use a 1/4" seam and the presser feet that are made for sewing these seams are very helpful to accurate seams. You will find that if every block is 1/16 of an inch off...after 8 that is a 1/2" if you are making small blocks for a large quilt, those discrepancies can really add up on each row.
Your pressing can also lose or gain inches. If you iron the seams they can stretch the block, if you don't press them open enough you can lose fabric in the seam lines...