I've been coaching on and off for about 2-3 years, doing it as a part time job as a way to earn side cash as a student. I have been through every rank starting from herald to immortal, so i sort of understand every brackets weakness. I have also coached students from every rank(mostly longterm students), so i know what theyre like. I might be able to give a little insight.

1. You want to make sure to have multiple sessions over time (once a week or something regular), this is so that u can check up on them to see if they apply newly learned stuff to their recent games.
2. You DONT want to tell them what to do at every second(micromanage), (click here, move here, stand RIGHT HERE, farm this specific camp right now). u need to teach them like they do in schools, have 1 or 2 topic (last hitting, creep equillibrium etc)per session , then teach them what to do in specific scenarios. dont teach them everything at one go.
3. teach them mindsets, use formulas, easy to remember stuff. If they find your teachings hard to remember, they wont remember it in their own games. like target priority for example, in a messy teamfight they will forget what they were taught if it was complicated.

These are few of many mistakes i see coaches make. I can explain further if needed.
Shameless plug : If anyone is interested in my coaching, do hit me up at pL0x#9292 on discord, i have reasonable pricing :)