So this is how i learned it.
First you must know how deep the water is and how strong te current is,make your leader(just 1 piece of mono i use) about 2 feet longer(about 50/60 cm).
When the leader is to long the fly will drag tomuch over the bottom and there will be a big curve in the leader,you will not see when a fish takes it,or to late.
When the leader is to short the fly wont reach the bottom,you will see a take but not much fish will take it.
When your setup is good but your stuck all the time on the bottom your fly is to heavy.
If you never get stuck on the bottom your fly is to light.
Cast upstream and start taking line back in imediatly,dont be afraid that you do this to fast if you do it to slow you lose contact with the fly and wont see or feel a take.
Just try to take line back in as fast as the current of the water,when the flyline is straigt or almost straight on the water your doing good,if your a lil faster a few times dont be afraid the fly will come lose from the bottom a few inches(cm) and makes it more attractive to the fish.
When fishing in some deeper pools just cast a few feet streamup over the pool,by the time the fly is deeper it will be in the pool.
When a fish takes the fly you will notice it,your indicator will stop or go under or you see the flyline make a short stop in the current.
Some ppl lift the rod then,i just give the flylne a good pull with the hand im taking in back the flyline,if its not a fish i can make the drift go on without casting for a new drift.
The rest is all practicing and by experiance you learn how long you have to make the leader and how heavy the flies have to be and where to drop the fly in the spots you wanna fish.
Klaas