when sewing strips together it is important to switch ends with each addition...so you put the first 2 together, then start the third strip at the end you ended on (flip-flop one end to the next) this helps keep your strips from (bowing) also, it is important to not (iron) your seams but Press them...no back and forth...lift, place, lift the iron. but most of the time the distortion is caused from starting at the same end with each new addition. really need to flip-flop then trim-square up one end to start cutting your squares.