The people to talk to about it are the racers at your track. There are different types of additives and they don't mix. Run the wrong additive and you will have less grip than using none, as the additive on your tyres will react with the additive laid on the track by the other cars.
The usual procedure is when you come off after a race you clean the tyre surface, again depending on what's allowed at your club. Most commonly 1:1 car brake cleaner is used to get the rubbish the tyres have picked up from the track off the tyres, some clubs won't allow something so strong so baby wipes are often used instead. Once clean you then use a suitable tyre additive to coat the tyres then add the warmers so bake it into the tyres. What additive you use, how much of the tyre you coat with additive and how long you leave it on for all makes a difference to how much grip you have and how the grip changes during the race.
Ask the fast drivers at your club what they use and how to treat the tyres, any decent racer will let you know what they use and give you some advice on how to use it.