It is all top secret, you have to kill a stuffed panda then fight Hulk Hogan....
In all seriousness, experience helps more than any casting qualifications, get to know the river and its moods, get to know the fish and the entomology of the insects, learn about what hatches in what season and at what time of day! This alone can take a few years.
Learn to cast, not just simple casts but all the trick slack line casts and mends that will help you cover the fish, now this is where getting a casting qualification will help, you will learn a lot and you will also begin to learn how to get this knowledge across.
Have plenty of kit, life vests, spare waders, rods reels, flies etc
Learn all the different techniques for different waters types, conditions and for different fish during the year
Get insurance, you will always get one person that ignores everything you say and they will try their very best to die whilst you are guiding them.
Drop me a PM if you want any more information