I used to have cars that needed bar's leak powder for whatever reason. Saturn s-series were one.I added a full gallon of Prestone which put him just barely above the full line and dropped in the tablets.
I'd take an old gatorade bottle, fill it mostly with 50/50 coolant, and the bars leak, and shake that up before pouring into a radiator with room for it to fit. Preferably with the motor running! That would suck it in and distribute it pretty well. If I'm unlucky to have a full system I drain an equivalent amount before working.
A lot of people put bars on top. It floats! This mostly clogs up the radiator cap and its valves and bypass hoses. The saturns used a surge tank that went into a heater/bypass hose, so if I could get the tank to be empty my evil concoction would immediately be swept up by the water pump and sent throughout the system.