From what I have seen from these sorts of queries over the years.

First one must have a set of skills. The ability to shoot well is really a given, and not the most important. A whole range of skills is often required including skinning, caping, stalking, tracking, riding, motor mechanic, cooking, knowledge of game, trophy estimation, local knowledge of the hunting area etc. What a guide or PH has to do depends on where they are and the setup. In Australia sometimes the guide is it, and does everything. Sometimes they have a cook/helper. In Africa a PH might have a cast of thousands, trackers, cooks, cleaners, waiters, skinners, butchers, drivers, gardeners etc etc to help them do what an Aussie guide may have to do alone.

As Phil says, people skills rank at the top. Nothing stops repeat business and word of mouth recommendations to other potential clients than having a useless, grumpy, arrogant, opinionated, lazy, idiotic etc guide or PH.

Secondly one way into the business is contacts. Getting recommended by a good client, relative, friend etc, may get one an introduction.

In Australia some guys end up guiding simply because they are good at what they do and are personable. As most Aussie hunters hunt pretty much 100% self-guided (a funny term here actually as it is an alien term) and 100% self-outfitted (again an alien term), what is the difference to guiding clients, other than doing it for someone else and being personable.

Failing the usual way some seem to get into the business by being an unpaid, or very lowly paid helper, doing all the shit jobs for a season or two, for next to no pay. Not living in grand client style accommodation but the hot airless tin shack around the back. Eating small portions at meals, only drinking juice or water at dinner - have a look sometime, often the apprentice gets bad looks from the boss if they drink a soft drink. Showing work ethic, friendliness, perseverance, team work, etc as well as the other skills. There is no doubt some get taken advantage of, but some also work through their "apprenticeships" and end up fully qualified and employed doing what they want to.