You have to make your fly behave like a fly (I won’t say think like a fly)
With a fly rod you cast it out and pull it in in jerking motions to get the attention of the salmon or trout or whatever. Then when it gets close you cast again, keep repeating til you get a bite
The rod and the line has to be very strong, its like hooking a speedboat and I aint caught anything as big as what you’ve got there 
The other trick is getting the right fly. Two ways of doing this: One is to catch and kill a fish, and use a priest to see what its eaten (basically a metal club for killing with a big scoop you stick down its throat). The other is to see whats flying around and try that
If you’ve got an old rod you can do it, its a bit tricky with a pole cos you have to pull the line in as you go. Make sure its strong enough and get a strong line for it. There’s a size regulation (over here anyway) during spawning season, where you can kill any fish over a certain size. Might be worth looking up