I want to install a 50 amp breaker box (exterior garage wall) in order to plug in our RV. I like to do things myself, but am apprehensive about undertaking this project. My main area of concern is wiring into the existing main box, which I believe is 200 amps at the main breaker. Any pointers/suggestions/instructions would be greatly appreciated.

My standard recommendation is that if anyone is not qualified, trained, and otherwise unsure of working in a circuit breaker panel, then a qualifed professional should be called in.

Any work done in a circuit breaker panel requires caution. Shutting off the main breaker will help, but beware, if the main breaker is located in that breaker panel, there still is 240 volts on the wires and exposed wire connections that feed this breaker.

If any work is done in a panel that cannot be completely shut off, then the worker must wear the proper protective gear, voltage rated insulated gloves, Arc rated face shield, and fire rated clothing. In the event there is a short that causes a arc flash, this flash will cause severe injury or worse.

If the main breaker is located remote, such as outside at the meter, then shutting off the main de-energizes the entire panel, and there should be no voltage in the panel. This is the only condition any person not qualified or trained should work in a panel.

Always test the equipment you will be touching with a good quality, working, votage tester, never assume because you shut off a switch that the switch actually worked.

There is also an important difference with the neutrals and grounds, depending on the location of the main breaker. If the main is in that panel, then all neutrals and grounds are connected together, if the main breaker is remote located, then the neutrals are separated and isolated from grounds wires..

If grounds and neutrals are not wired appropriately, additional issues with safety arises with the entire electrical system.

Not trying to scare the bejezzus out of you ,only trying to get a point across to help you, and others, decide if this work can be done safely.

Hope this helps, get back with any other questions you may have.