Manual Select Method Multi choose- Option 1
When you click on the competitors page you will notice your activated products list and in the right column the competitors tab. Click here on the listing to set competitors manually.
By Clicking on the link in the competitor column you are taken through to the next page below. Here you can choose competitors. Pressing the 'Click here to deselect competitors' toggles you through seeing selected and unselected competitors.
Manual Method by Listing ID - Option 2
From the competitors page.
Begin by Clicking on the green button 'Add competitors manually'.
This takes you through to the next page where you can add products by their ID.
eBay product numbers can either be found on the eBay page in URL or in the top left of the description box. Examples of both are shown below.