I rode Western for the longest time before switching to English, so I understand how confusing a post can be.
The biggest piece of advice I could probably give you is don't over-think it.
I did that and it was a big mistake, haha. It made everything a lot more confusing than it had to be.
First I would practice posting at the walk. It's harder at the walk because your horse isn't pushing you out of the saddle as much but you can work on the movement without really having to worry about speed or diagonals like you would at the trot. Make sure to post with your leg and not off of your stirrups.
I would try posting at the trot without worrying about diagonals when you're just beginning.
Remember to keep pace with your horse. If you go faster your horse will, too and if you go slower so will your horse (this is helpful to remember when you would like to adjust your horse's pace).
Remember to breathe (I still hold my breath so don't feel bad if you have trouble
For diagonals "Rise and fall with the leg on the wall".
It will take a bit to get used to but have faith!