The way I teach my students is to count. The trot is a two beat gait, so when a horse trots, the beat is 1-2-1-2-1-2. Take a few strides and find the 1-2 rhythm. Then count with it, either in your head or out loud, then rise every time you say 1, and sit when you say 2. Worry about which shoulder you are following once you have the rhythm down.
A LOT of new riders (and some more experienced ones) overpost the trot, and rise higher than they need to. Once you find the rhythm, don't work so hard at it, let the motion of the horse push you up. Try not to rely on your stirrups for anything more than a "location finder" for your foot, to help your leg be in the right spot. Don't brace on them, use your leg muscles and core to do the work, not your feet. (That's not as easy as it sounds, of course. That's the part that takes practice, often years of it, to get right.)
"Keep a leg on each side and your mind in the middle"