To teach any horse any move of any kind you need to be able to do it yourself on an experienced horse first. Old horses for young riders etc. There are various levels of dressage competitions.
Leg yield is not a recognised movement in a competition but is a schooling movement, it is used to ask the horse to move away from your leg in a sideways and forward movement and not just forward . To execute this movement you turn up the three quarter line in the school ride straight for a few strides and then move your outside hand away from the horses neck, be careful not to pull him over. At the same time move your inside leg slightly back and put a small amout of pressure, increase this if no response. The horse will initially run on forward as this is what he is expecting to do. Check him with the rein and re ask. If he turns his head to the outside balance him with the inside rein. He needs inside bend and he should smoothly move over keeping balance and rhythym.
Shoulder in is a more advanced movement. To execute this correctly your horse should be rounded over the back, actively stepping forward and remaining soft in his mouth. He should be attentive at all times. To execute shoulder in - instead of passing c and riding on around to m turn slightly earlier so the horse is almost heading on a diagonal back to h, hold you outside rein so the horse doesn't turn too far of the outside track but use your inside rein to encourage him to stay slightly of the track, put your inside leg slightly back and encourage the horse to travel sideways but forward. The shoulder in is ridden on 3 tracks. Inside track is your inside fore, 2nd track is outside fore and inside hind and the last track outside is the outside hind. This movement takes time to perfect and you will probably benefit from a helper on the floor.
Lengenthen strides shouldn't be attempted until your prelim horse is remaining, balanced, forward, soft and rounded through all transitions, he must be remaining straight on straight lines and bent correctly on turns. Ride in a steady rhythym at all times count quietly to yourself, watch your horse is still moving forward actively, not rushing. If you perform this movement too soon you will unbalance your horse and ruin anything he has learnt so far and you will also loose his confidence. To perform this correctly your horse needs to be able to collect and extend with a strong back.
Early collect training for dressage is important and there are no short cuts, the more correct effort that is put into the horse the better the outcome. If you rush things the horse he will not become truly supple and able to carry out the movements when you ask.
You would be well advised to get hold of some basic prelim tests maybe even walk and trot tests and practise some of these? Get yourself a good instructor not just any instructor but someone who specialises in dressage at least up to Prix St. George level. Be patient!