To avoid a bump - lay out your pieces ( I use the floor for this) with one slightly overlapping the other. Take your trusty scissors and cut through both pieces - keeping the scissors vertical while doing so. Take out the resulting scraps and - well, then I hand stitch them together with a big basting stitch. I've never had any trouble with stuff shifting. It's also good exercise to crawl around on the floor. Produces a nice bottom line so to speak.