Teaching new setters is hard. Even experienced routesetters who are great at what they do speak about the difficulties in breaking down setting, and turning psyched climbers into efficient, effective routesetters. Here are a few tips that might help out.
1. Explain What Setting Isn't
This is the first thing you need to do. Day 1. The majority of people who are interested in starting setting are strong, psyched climbers, who want to create fun problems and routes for themselves.
Make sure that the new setter understands that their role is to design a climbing experience for customers - and not for themselves.
2. Let Them Make Mistakes
When somebody is new at something, they're not very good. That's how skill acquisition works, and routesetting is no different. The routes and problems that new setters put up and generally not going to be very good. It's hard as an experienced setter (especially a head setter) to see a sub-quality product go up on the wall of your gym, but it's an important part of the learning process.
If you pull them down, or totally take over the tweaking process, you'll crush the confidence of the new setter, and they won't learn. Help them tweak the route, and explain why every step of the way.
3. Establish Gym Setting Principles
Before bringing on new setters, establish the routesetting principles of your gym. What are you trying to do when you set routes or problems? Provide a varied experience? Teach people how to climb? Get it down on paper.
By having these things formalised, you can reference specific principles and guidelines when explaining tweaks that need to be made, or the sort of route you'd like the new setter to put up.
4. Easy Routes First
Make the new setter set easy routes or boulders first. Easy climbing has simple sequences, and it will allow the setter to focus on rope-work, learning how to use the drill, and all the other things they need to learn before they worry about how to create complex movement.
If they're a strong climber, it will also help re-enforce rule number 1 - you're not setting for yourself.
5. Pay Them
I almost can't believe this point needs to be made, but given the state of the industry, it does. If you want someone to do something professionally, and to be able to hold them to account, you need to pay them. Your routes and boulders are the product you're selling at a gym - why on earth would you want have the people responsible for creating that product be volunteers?
Yes, good setters take time to train. It's an investment. Like any staff, treat them well, compensate them accordingly, and before long you'll have a team you can rely on.