Thread your tails onto a needle (a self threading needle works very well) and insert the needle into the same hole where the knot is, or just behind it.
Run the needle inside the layers of your quilt sandwich. It is a good idea to double check the back side to make sure your threads aren't visible there. Push the needle to the front of your quilt about an inch away from your starting point.
Pull your thread ends through and give them a gentle tug. Make sure the knot pops below the surface of the fabric. Use a scissors to trim the tails.