press hst pieces while they are folded- with light side down to set the seam, then open them and press (so the seam will fold toward the dark side- press---do not iron back & forth which will stretch the bias edge & cause the half square triangle to be wonky. generally patterns tell you what direction to press your pieces/blocks- follow those instructions to achieve the best results. always press (place iron down flat, lift, move, press) do not iron across pieces/seams.