Foreign income earned prior to arriving in Canada. We want to know about your US gross income. It boils down to whether you get a full year or is it prorated from your date of entry into Canada. Therefore, you are not taxed twice based on the tax treaty we have with the USA.
You would apply it under the FTC form, which can be located by clicking on the “Find” icon at the top right of your page and typing in “Foreign Tax Credit” this will bring up the correct form to use to claim the taxes paid.
As for determining how to calculate your taxes paid for April to December, I would suggest you contact your employer to figure out the taxes paid for that time period.