You never want to line up a seam with the center of your machine. Generally, you would take a full width, center it, and add pieces on either side to make the backing. Everything you put on your machine should be centered on the rails. I permanently marked my rails center. Then each piece I put on I center with that point. Your top rail is your take up rail. The next rail is backing, the next is your top. I use cut pieces instead of putting a roll of batting on the machine so I slide it up between top and backing, smooth it put and pin along with the top's edge. I have a vertical lock on my machine so I can sew long basting stitches across the top to hold it in place while quilting, Hope this helps. Look for a tute on utube. There are many out there on Loading your quilt frame.