What is the province that the merchant is in and what provinces do they want to charge tax for? This is the first step, you must have clearly defined criteria to setup the tax rules.
Once you know exactly under what circumstances and the value to charge then setting up the tax rules will be very easy.
The general idea is that on the page that your checkout page posts to (usually the confirm page) you will want to add a 'set session value' server behavior. This server behavior will be used to create a new session variable, you can call it province. In this variable you will store the value from the posted province select list on the checkout form.
Once you have the province held in a session variable you can create the charge rules based on your criteria. You would first want to check for the session variable existing, then based on its value. You would have a rule like this for each province that you wanted a different tax rate applied to. For the amount you would base it off of the cart subtotal and multiply it by the decimal tax rate, so for 8.5% it would be multiplied by 0.85. Please post back with any additional questions that you have about this.