You actually don't want to go to the full concentration at once. That dose is the correct one for treating ich (0.3% salt). It is quite high & fish should be acclimatized to it to reduce stress.
I normally up the salt over 2 days. So put 1/3 of the amount in now, 1/3 in 24 hrs, and the final 3rd the day after. You want to pre-dissolve the salt in some tank water before adding to the tank. Since solid grains of salt can burn the fish if it comes into contact with the skin.
If you go the salt route, remember to keep the salt level constant. If you are doing pwc's, you need to add an appropriate amount of salt to the change water. Also, salt don't evaporate, so it will concentrate in the tank if you get a lot of evaporation. The safest way to limit this salt creep is to mark your water level. If your tank is at the water mark at pwc, no worries, just add the same amount of salt in the change water. If the level is lower, then you add fresh water to the tank up to the mark BEFORE you do the pwc. This will dilute the salt back to the appropriate level so you remove the appropriate amount of salt with your water change.
You keep the 0.3% level of salt in the tank for at least 2 weeks after all signs of ich is gone, then drop that to zero over the next few weeks with 10-20% pwc's every few days.