I have always used size 10 crochet thread, doubled. I use a large eye needle used with yarn with a sharp point. I put it down one side and up about 1/4 inch away from the down side. Then I go thru the same holes a second time (down and then up). I do a square knot. I say to myself as I tie the knot~ right over left.....left over right ~ I have washed my first one done this way every week for several years and the knots have not come undone. Hope this helps.