What an exciting time for you with the grandchild on the way. I'm sure that whatever method you use to make this sweet quilt the baby and his mom will love it. The only thing I can think to add to everyone else's suggestions is that before you sandwich and start sewing, it would be a good idea to iron both the top and the back using quite a bit of starch. A few minutes in the clothes dryer will take the wrinkles out of the batting (if it is cotton). This will help you a great deal in keeping everything straight. After it's done you'll want to wash the starch out of the finished quilt before using it for the baby. If you do decide to bind it, you would probably find it easier to manage a bias binding. This means the fabric is cut in strips across the diagonal. It will be more flexible than the straight trimmings from your backing. You might have to buy more fabric, but probably less than a yard. You could use a contrasting fabric, such as a solid color from the design. You should be able to find good instructions for binding both on this board and on Youtube, or perhaps there is an experienced quilter near you who could guide you through the steps initially. It's much easier to show than to explain. Best wishes for the wee one to be.