Hello! and welcome. What I'd do--I haven't constructed a quilt this way but have thought about--is assemble it pillowcase style. You make the front and back precisely the same size and sew the 3 layers together before turning and quilting. You don't attach a binding. (This is just a sketchy outline; detailed info is available here and elsewhere on line.) However, if you want a binding, you can assemble the sandwich of front, identical-size back and batting, and then baste it very densely so nothing can move--then bind and quilt. In any case, you'd have to make the pieced backing the same size as the front so the borders will be as they should--can't see any way around it. But you know what? it's your first quilt; give yourself a break and just use a plain back. :-)