When you come to a stop, pull up the bobbin thread, leaving long enough threads (bobbin and top thread) to hand tie a knot close to the fabric where you pulled it up. Then, thread the two threads through a needle. Put the threaded needle down through the same hole as your knot and go between the layers for a couple of inches and pull it back up. Pull on those threads until your knot pops down underneath the top layer and then cut off the threads that you pulled back up. I hope that makes sense to you.