How to create Marketplace-specific SKUs/listings

I’ve been having issues for the past 2 months when I’m trying to add FBA listings to our Canada Marketplace. The listings are there (with a different SKU) but the inventory from the US FBA listings syncs to the new Canada listings. I’ve raised heaps of cases with Amazon about it and one mentioned using Marketplace-specific SKUs, which means you can manage distinct pools of inventory. I’m not sure how to create these, can anyone help me?