After adding fuel injector cleaner to your gasoline tank it gets picked up by the fuel pump within at least a few minutes of the engine running.
If your car is hard to stay running you should let the engine run at idle for around 5 minutes to allow the fuel injector cleaner to get through to the fuel injectors.
After 5 or 10 minutes of the engine running the fuel injectors should begin to get cleaner and the engine should start running much smoother as a result.
If the fuel injectors are really clogged or really dirty then you might need to have the fuel injectors professionally cleaned at an auto shop.
The auto shop will use a high powered pressurized system to help clean the nozzles and the fuel injectors out.