Bring both ends of the thread to the top of the quilt, tie the ends in a double knot close against the quilt, thread both onto a needle (using a self-threading needle is helpful for this), slip the needle into the quilt sandwich right by where the threads come up, run the needle over as far as you can into the quilt sandwich, bring it back up, pop the knot into the quilt by pulling gently, then snip off the ends of the threads near the quilt top and let them disappear into the quilt.