I've used perle cotton and embroidery floss. I used yarn in a flannel baby quilt and didn't like how after many washings they became little balls. When tieing use a long strand of your string and take a little bite of the sandwich every 4 inches in a line across the quilt. Do not cut the string until you get to the other side. You can then go back and cut between the 4 inch spaces and cut the string and you will 2 inch tails to tie in a square knot.