A thin rod — two old clubs do the same job — laid on the ground to check your target line against your foot line. The fastest cure there is for crooked shots.
Most crooked shots are created before the swing even starts: your shoulders aim somewhere your feet don't, and you never notice, because you stand side-on to the target and your perception lies to you. An alignment stick settles that before you hit a single ball. Lay one rod just in front of your toes, parallel to the target line, and the second one beyond the ball, on the target line itself. Target line and foot line now sit side by side like two railway tracks — they do not converge on the same point, they run parallel past it. That is what almost everyone gets wrong: as a right-hander, aiming your feet straight at the flag actually aims the ball right of it. Two old clubs work just as well. And since correct alignment feels wrong at first, trust the stick, not the feeling — after a few sessions the feeling catches up on its own.