The envelope, or pillowcase, method works fine for me. I layer the pieces as described, making sure there are no wrinkles or puckering in the layers, then pin;
stitch all around; trim off any excess beyond whatever seam allowance I want (I state it this way because often with these turned quilts I leave a full 1/2" seam allowance, so when I turn and topstitch 1/2" from the edge it makes a pleasing filled edge);
turn the whole thing right side out;
top stitch and it's all good to go.
The easiest way I've ever seen.