You can have a go without training, watch youtube videos and ask as many questions as you can but if you want to take it seriously then i would advise taking a course, it is nothing like plastering and without experienced tuition you may never understand the concept and would waste a lot of time and money.
Goldtrowel have a good reputation for these courses offering more than just a few sample boards and covering everything from preperation to real world situations. Corners etc.
Even after completing a course it still takes a while for it to make sense so practice as much as you can.
There are hundreds of techniques that produce different finishes, you need to 'forget' how to plaster (gypsum) and use random trowel movements,
There is a growing market for these finishes and everybody who uses them actually enjoys their job!