Redex One Shot Petrol Injector Cleaner 50ml comes with full instructions but I believe you just add it to your fuel tank.

add half of it to a full tank,then the other half to the next half,please never let the petrol tank to be completely empty,

Redex One Shot Petrol Injector Cleaner can be added every time you fill up the tank. The more frequently it is used the better the performance of the engine in terms of fuel economy & emissions.

First of all, one treatment of Redex isn't going to make much difference, it is designed to be used regularly (every tank fill). Redex won't raise emissions, especially if only used once. It may be able to help with the misfire, if the misfire is related to the fuel system (e.g. cleanliness of the system). If the cause of the misfire is not related to the fuel/fuel system (spark plugs, timing, air filter, engine electronics...), Redex can't help with that.

It's not recommended for 2 strokes. It wouldn't damage the engine but there is no proven benefit. Two stroke engines do not have valves so there is less for it to clean.