If you want to do the 12 hours thing, it's 1 tsp per gallon, then 12 hours later, add another 1 tsp per gallon, then 12 hours after that, add another 1 tsp per gallon. this will bring you to 3 tsp per gallon total. You could go to 3 tsp per gallon right away as well, but with some sensitive species like plecos it's often better to ramp it up slowly.
Don't do water changes til you are at 3 tsp per gallon. You can do changes after that but you will have to re-dose the salt you took out. If you take out 10g water, you will have to re-add 30 teaspoons of salt with the fresh 10 gallons of water. Remember to disolve the salt in water before you add it.