Link to the page for the exact product, in the size or pack you sell. Not a category page, not search results, and not the brand's own page.
Sizes are where this goes wrong
A competitor selling the same food in 2kg and 12kg will have two pages at two different prices. Link the wrong one and you'll show a real price that isn't comparable to yours. This catches people out more than anything else.
Pages covering several sizes
If one page covers several variants at different prices, we refuse to show anything and tell you so, because we cannot tell which one matches yours. Where the competitor is on Shopify you can usually fix this by choosing the size on their page and copying the URL from your address bar: it will end in something like ?variant=12345, and we will read that exact variant.
Barcodes help
Products with a barcode in Shopify are far easier to match reliably, and barcodes are what future automatic matching will use. Filling them in is worth the effort.