Hi MomaDragonfly, got your message. Technically, you can add the salt straight to the tank but it's usually best to predissolve it in a bit of tank water first. It doesn't matter how much water so long as it is enough for the salt to dissolve. It can take a looooong time for the salt to dissolve thoroughly so you may want to dissolve it as much as you can stand and then pour it in the tank in a corner away from where your bettta is.
Just to reiterate, the dosage to treat possible external parasites is 3 tsps per gallon and increase the heat to 86 F. For a normal dose to treat wounds or fin damage, it's 1 tsp per gallon.
When you do water changes with AQ salt, it is important to never put in more salt than you take out. So, for instance you said you do 50% water changes? Then you would be removing roughly 2.5 gallons of water. If the dosage of salt you are using is 1 tsp per gallon, then you would fill the tank with 2.5 gallons of water and add back in 2 and a half tsps of salt, NOT 5 tsps.
The rip looks clean and it will probably heal within a few days with warm clean water and a high protein diet.
If you use the 3 tsps dosage, remove your live plants as they most likely will not surive the salt water.