If you have a large flat surface you can mark the beginning line and the end line in the seam allowance. Place the painter's tape edge on the bottom mark and stretch one long piece to the mark on the other side. The painter's tape pulled taut should act like a chalk line and be straight. You will have to fasten the quilt down so it doesn't move when you stretch the tape across the top. All subsequent lines can be marked from the first one?