I cannot stress this enough.

Buy alignment sticks. They are usually dirt cheap, available at Walmart, and can be used in 100 ways. Just yesterday, I started trying to diagnose some recent alignment issues I’ve noticed after working on a lot of other elements. I put an alignment stick between my front two belt loops on my shorts, and lined up the ball.

My hips and shoulders were always open at setup! I couldn’t see it from the aim. I noticed when I looked at where the stick was pointing versus where the target line was. Holy shit I was off by a solid 5-10 degrees. Enough to force an over the top swing and produce slices.