The Double-Strip ruler is one of a range of strip rulers designed to make cutting 1/2-Sq-Triangles, 1/4-Sq-Triangles,60 degree Triangles, Trapezoids and more from 2-1/2in wide strips quickly & easily! One side is used to cut your strips & the other side has relevant triangles.
Adele's Tips: Use starch to prevent stretching.
Try Using your Sew Steady Grid Glider line to get better alignment for those 60 degree seams.
Also, trimming corners [Marti Michell Deluxe Corner Trimmer] might help with bulk and alignment.