Here I have devised a step by step model, to help teach young riders the process of riding on the correct diagonal. I find this works so well as it uses visual, auditory, and kinesthetic learning styles. You can teach it all at once, or if you are working with a younger rider on a regular basis, you can break it up into chunks.
1. Ask the rider to say 'now' every time the outside front foot hits the ground in walk. They may look down over the horses outside shoulder to do this. Ask them to imaging the hoof as a drum, and it makes a noise as it hits the ground.
2. Ask the rider to say 'now' every time the outside front foot hits the ground in trot. It is important that the word 'now' is sounded out short and sharp, as to not merge into the leg moving forward.
3. Ask the rider to say 'now' every time their seat (bottom) sits in the saddle. Again ask them to imagine their seat making a drum noise as they sit in the saddle. Once they have their head around that, you can ask them to do the same but when they are at the 'top' of the rise. This is much harder, so let it be OK if it takes them several attempts before they are comfortable with this.
4. Next, whilst the rider is trotting clap your hands once with a short sharp clap every time the rider either sits in the saddle or is out of the saddle (the coach chooses) in rhythm to the trot. Ask the rider to identify if when they hear you clap are they either sitting or rising. Do this several times and change from sitting to rising. Again, when you clap as the rider is sitting they pick this up quickly, but often takes longer when you clap as they are at the top of the rise.
5. Let the rider know we are going to join the two together now. Ask the rider to again say 'now' every time the outside front foot hits the ground, and then ask them to identify if they are sitting or rising when they say 'now'. You may need to clap occasionally as the outside front foot hits the ground to help them out in the beginning.
6. Ensure your rider knows how to sit for two beats to change diagonal, keeping their chest open, core strong and hips swinging.
7. Lastly, fit everything into place :) Ask your rider to say 'now' when the outside front foot hits the ground, then ask then to identify whether they are sitting or rising as they say 'now', and then if they are rising ask them to sit for two beats, if they are sitting then they are correct.
8. Once they are confidant doing that you can have a bit of fun with it. For example ask the rider to sit for a few strides and close their eyes (on the lunge of course) and then let them open their eyes, and see how quickly they can work out what diagonal they are on. Another fun one is to ask them to sit for two beats, then three, then four, and so on and so on. This helps them to feel the rhythm, builds their confidence in sitting trot as well as beginning to feel the footfalls in the front feet.