Okay, maybe I did something terribly wrong as a beginner, but how did you make sure that your posture is good?
I used to do the exercise with little a no weight, record myself and double check for a longer time, until I found that I got used to the right movements.
At the beginning you should be on a relatively light on the weight anyway and as you progress you should be building locking in that form naturally.
Sometimes you'll need to deload to correct form and you'll likely be starting to look at different programs soon enough.
For one reason or another I've had to deload, usually missing training for a prolonged period of time. I'm at my longest sustained training period in awhile now and about to attempt a new 5rm squat tonight, I'm a little intimidated:). It soon becomes more about the psychology imo.
Video occasionally but once I knew what good posture felt like then I could feel when I was leaving it.
I always found good lifting form to be a will power thing more than a skill thing. Easy to maintain when the bar was empty, very hard to maintain on the last couple reps of my heaviest sets. Did form check videos occasionally to keep myself honest but really, I knew when I had bad form.
I used to do the exercise with little a no weight, record myself and double check for a longer time, until I found that I got used to the right movements.