if you are using cotton batting, place one piece on top of the other so the edges overlap slightly. have your cutting mat underneath the pieces. Using your rotary cutter, cut a *wavy* line. This creates an edge which is very easy to keep together. Take the very edge pieces off and you will have 2 main pieces that fit together like a jigsaw puzzle. Take them to your machine and use a 3 stitch zig zag. you can set your machine so that it is very loose, rather than a tight stitch. It will go together without lumps and will be fine to use.
Hope this helps.
Lisa