To get rid of a canker sore using baking soda you take about a 1/2 cup of warm water and dissolve 1 teaspoon of baking soda in the 1/2 cup of warm water.
Then you just rinse your mouth with the baking soda and warm water by swishing it around making sure to get the baking soda and water in contact with the canker sore.
You can also get rid of a canker sore by dabbing a little bit of milk of magnesia on the canker sore a few times per day.
Regular orajel or baby orajel can also help get rid of canker sores and will help numb the canker sore too so you'll be able to eat without pain.
Do avoid eating anything that is spicy or acidic when you have a canker sore to avoid your canker sore getting further irritated.