Select a Business Manager account from the drop-down (Make sure that you have admin access to the commerce manager)

Select a pixel from the drop-down. This will be used as the event source for the catalog.

Select a Google Product Category to be used for all products. You can type a keyword to narrow down the list, and then pick the category that best fits all your products.

Select "Include All Variants" if you also want to include variants with the product

Select Product Image/ Variant Image/ Alternative Image

Select "Include Quantity to Sell" (For more details follow the article: https://help.redretarget.com/en/articles/6441151-include-quantity-to-sell)

Give a catalog name and Click “Create catalog” to create the catalog