You will need to determine the proceeds of the sale (less any transaction costs) and subtract your Adjusted Cost Basis (ACB).
Your ACB is usually the amount you paid for the asset (including transaction costs) but can be adjusted by various factors. If you own multiple identical properties (i.e. you buy shares in the same company at different times) you would use the weighted average cost basis.
For each purchase and sale, the amount should be converted to Canadian dollars at the exchange rate at the time of the transaction.
Monthly or annual exchange rates may be used if the transactions occur over a period of time.