How do I fill the "foreign slip" section? I've already paid taxes in the US so I don't want to accidentally have double taxation.

You will not be double taxes as Canada has a treaty with the US. 

As long as you follow these rules- it will be entered correctly

• If the amounts on your foreign income slip are reported in a foreign currency, convert them to Canadian dollars before you enter them. If the amount was paid at various times throughout the year, use an average annual exchange rate.
• For foreign non-business income that is not on a T-slip (including US social security), do not convert the amounts to Canadian dollars before you enter them. TurboTax converts the amounts automatically based on the exchange rate you enter.

Here you can find the exchange rate.