Go and ride a few fifteen meter circles or a turn into the short side of a ménage. If you want to learn diagonals then trot the circle on the right diagonal and then once on the wrong diagonal. Keep doing that. Don't make it a habit to look down. It will take forever to actually learn feel. You will start feeling the "wrongness" of being on the incorrect diagonal. As for sitting trot-that's only so that when you start trotting you get the feel of the correct diagonal. It helps but after a while you will automatically just start on the right diagonal without knowing.
I'm pretty sure you sit 3 beats then start posting but it doesn't work everytime
I usually look at the outside shoulder when that leg moves forward you should rise and when it moves back you should sit but sitting for two beats would work aswell.