I use it just like prime. I add it to the tank just before I add the new water after a change, but I dose for the entire tank size rather than the water I'm adding because I read that the ceo of Seachem advised I'd somewhere.
It's really hard to dose safe accurately for a smaller tank, but, I know you can make up a solution of prime with it that will keep for 30 days, so perhaps someone else can chime in with the correct ratios to do that as I have no idea.
I don't worry too much about overdosing, I use a 1/4 tsp full as it's the smallest measurement given on the pack, and my tap water is literally full of nitrates and ammonia, so any excess helps to detoxify that once its dealt with the chlorine.
Safe is a reducing agent, so once it's munched through the chlorine and chloramine, it will start on the ammonia, then nitrite, then nitrate till there's nothing left, but be careful as if you overdose too much it can start reducing your oxygen if it eats through all of that and there is still some left.
Safe is a fantastic chemical. Don't be scared of it. It's a must have for people like me with terrible tap water, and also, a seneye as if an ammonia spike appears I can take quick action.