It is important that you do not throw fish back into the water as they are fragile creatures and could be damaged. Instead, gently slip the fish into the water nose first and it should swim away strongly.
On high banks you may not be able to reach the water, so return the fish by lowering it to the surface in your landing net and gently turning it out.
Some fish, notably Barbel, need time to recover before they can swim away and you may need to support them with their noses pointed into the current until you feel them swim away from you.
If you use a keep net, release the fish at the end of the session by gently lifting the end of the net furthest from the mouth so the fish swim out. Do not allow fish to flap around in the bottom of a net out of water as the larger fish will damage themselves, and other smaller fish below them.
If you want to weigh your fish after a session you will need to remove them from the net while it is out of water. Some keep nets have a ring that allows the bottom of the net to be lifted out of the mouth so fish can be removed safely. If your keep net does not have this feature, move the fish as near to the mouth of the net as possible using the method in the previous paragraph before gently tipping them into the weighing net. The head of a landing net can be used to weigh fish if you do not have a specialist weighing net or basket.