You want to map as many similar source products to a harmonized product as possible.
You use a layout with the following assignment blocks:
Search Results: Harmonized Products
Search Results: Source Products or Proposals: Source Products
Work Area
In the assignment block Search Results: Harmonized Products you search for a harmonized product and add it to the work area.
In the assignment block Search Results: Source Products you search for similar source products, or you update the proposals for the selected harmonized product in the Proposals: Source Products assignment block.
You select all source products that can be added to the mapping and choose Assign to Harmonized Product in Work Area.
You save.