no you dont you can be from lanc-corpral and above i believe. but you have to have a lot of experiance with the marines, like at least about 10 years service i recon. But most people i have talked to who train recruits tend to have like 15+ years experiance in the marines.
and i think you just apply for it really, like most specialisations. apart from when applying to become an instructor you would probably be put through tests that are going to be a hell of a lot harder than other tests as you are teaching future royal marines.