As I understand the process once you decide on one method ie using an avg exchange rate or actual rate on day of the transaction you have to use the same method for all transactions presumably forever.Here's how I do it:
Look up the exchange rate for the date your security was purchased on the Bank of Canada Currency Lookup website.
Calculate your buying price in C$ using the exchange rate determined above.
Repeat process for the sell transaction.
(Selling price - buying price) is your capital gains / loss.
You could also try the average exchange rate for the year you purchased and sold and use the method that is advantageous to you.