If you wanted to do work on HVAC installations as a full-time job then it would be very sensible to go work with a company that specialises in this sort of thing for a couple of months. You could try taking a course and going it alone, but it's unlikely to teach you everything you need to know.
I'd like to make clear that HVAC is not my daily job and therefore I probably shouldn't be answering your question. However, I'm not sure that many people on here do meet the criteria, so I'll try my best.
You'll need some experience (and assistance) in installing the outdoor condensor units. They can be very heavy and it would make sense to learn the best ways to move these into place, and how to go about safely mounting to walls and roofs.
You need to gain a few metalwork skills that will help you when installing cable tray for the refrigerant lines and power cables. You'll need a formal qualification for the electrical work, and if you're going domestic then you will also need to be part P registered or notify all work you carry out to the local building control.
I imagine there's also a lot of brazing and pipe bending required, correctly torquing up compression, plastic pipework for condensate drains, etc. Then there will be some maths involved in working out heat loads and choosing the correct sized system to install into a building.
You'll be working with refrigerants so will need the correct equipment and training in charging and discharging HVAC systems, I'd say this is pretty important although can't say if any formal qualification is required. If you're working in an industrial environment then you'll want to know about VRV/VRF and ducted systems too.
Hopefully that's a start. Now to somebody who does this day-in-day-out who can tell you everything above is wrong
