I would premix it, as Bombalurina recommended. You don't want the fish to come in contact with the salt crystals.
Whenever you do a water change, only replace the amount you removed (not the full amount). For example: Since you're treating the whole 10 gal tank:
- If you do a 20% water change, you'd be removing 2 gallons. So mix 2 teaspoons of salt into the 2 gallons of new water. Stir until dissolved. Add it slowly to allow time to acclimate.
- If you do a 50% water change, you're removing 5 gallons. So mix 5 teaspoons of salt into the 5 gallons of new water, and stir to dissolve. Acclimate slowly.