start trotting while sitting, then when you think you feel the right diagonal, start posting
I could for 6 beats then start rising
You'll develop a feel for it with time
I've found that putting tape on the shoulder muscles of your horse is easier to see when trotting. Of course not any tape that would hurt your horse or anything but it's just a little thing to try while learning the feeling of it. We all struggle with this so don't beat yourself up over it! After time you simply start to feel it and soon enough you won't even have to think about it :)
i had a very hard time getting the right posting diaganal when i first started so don't feel bad about it! it takes time & it's something you need to practice lots
Practice! Diagonals take time, and the more you practice the easier it will be. I found the best way to do it was to go into the trot and post a few strides without looking, trying to feel if you're on the right diagonal, then say out loud what you think, then check and reward yourself if you're right and go back to walk or if you're wrong then change and post a few strides on the correct diagonal.