Lots of good advice but I would advise against open seams. They are a disaster waiting to happen when trying to line up seams. If you think through the project and iron your seams the correct way, all your folded over seams will lock against each other when you are sewing two adjacent rows together. The method of locking seams as you sew even works with slight discrepancies in block width when you are sewing. You can get the hang of locking the upper and lower fabric together and find out how to make your sewing machine feet even out the fabric as you sew down a line of connections. Much easier to show in person than describe here.