There are a number of different routes, and much of it surrounds where you'd like to fish: freshwater or salt?
I personally guided saltwater for years up in the Haida Gwaii (Charlottes). Most of the folks I knew who guided up there got in one of two ways:
1) Got hired on as dockstaff and over the course of 1-3 years, proved their worth and got "promoted" to guiding
2) Cut their teeth as guides down here closer to Vancouver, and then after building experience applied for and got hired on as a guide up north.
Both take years and can be rough, but rewarding (mentally rewarding, though generally not monetarily rewarding; it's a lot of work for moderate to poor pay, believe it or not).
I was fortunate and got hired straight on as a guide, but that was after fishing up there one trip and getting to know the fishmaster really well.
For freshwater, the avenues are different. Someone else here can probably speak better to this topic, but basically you can be an assistant freshwater guide (under a head guide, usually as part of a guiding outfit/company), or you can be a head guide and run your own operation (have to get properly licensed, and this takes $$$ I believe).
Remember, guiding *can* be seasonal, but if you're willing to build relationships, move around, and fish different areas/countries, you can do it year round.
You'll probably never get rich, nor even get overly comfortable from a financial standpoint, and your relationships may suffer (long-term girlfriends/boyfriends/spouses + guiding RARELY mix well), but you WILL have some of the most incredible experiences of your life.
Good luck!