If you haven't yet created your promotion, see Adding a Promotion.
Once you've created your promotion you will need to assign at least one product to it. To do this click on the "Assign products" tab.
In the box under"Select products & categories to assign", start typing the name of a product or category of products you want to add to the promotion.
For example, you might want to run a promotion on Corona.
Or you might want to run it on the entire category of bottled drinks.
Once you have added a product or category to the promotion you will see these under "Assigned products & categories".
Note if a product has measures, you can select which measures are part of the promotion.
If you want a promotion to apply to all of your products, just click "Assign to all products". You will then see a message indicating that all products are assigned to this promotion.
If you want to run your promotion now, just press "Done" and then "Sync changes".