the quilting needs to be done before you cut the backing the same as the front- or it will wind up being smaller than the front when you quilt it---you layer the (sandwich---backing, batting, top---baste either with pins or with thread or with a spray basting----quilt it---then square it up- trim it even and add a binding-
or you can fold excess from the back to the front and machine stitch it as a binding---or you can put the top & back facing---stitch around 3 sides- turn right side out----quilt- then finish the final edge---but if you are going to bind it you need to quilt it first---and the backing needs to be larger than the top to quilt it