Try using a "cheater needle" They're the ones that you can snap the thread into the eye instead of having to thread it through. Once you've tied the threads - either a square knot or a quilter's knot, thread the ends into a needle. Pull it up slightly so you can see exactly where one of the threads goes through your fabric. Put your needle through that hole and slide along under neath the top for an inch or so and then let it come back up. Pull the thread through. When you get to the knot, it will "pop" as you pull it into the hole. Sometimes you might have to manipulate the fabric a bit if the knot is big. Once the knot is buried, you can cut off the thread where it came back out of the quilt top.