When you are quilting pieces for clothing, you have some choices. (1) You can quilt a piece large enough to cut your garment pieces, then cut out and assemble the garment.  Be sure to allow for shrinkage from quilting. (2) Cut pieces a bit larger than the pattern, then quilt, cut to size and assemble.  (3)  Partially assemble the garment, then quilt over seams.  Be careful to keep the piece as flat as possible to minimize shrinkage.  You may want to cut the pieces slightly larger than the pattern to start.