1) Login to your AppSell dashboard.
2) Click on the "Create a new funnel" button:
3) Select the "Cart page" type and set triggers for showing your funnel:
Click on Next:
4) Select any section from the page and click on "Add widget" on the left side bar:
5) Select the Product upsell widget:
6) Click on "Select Product" and select the product you would like to offer as an upsell:
7) Change the design, timer, discount properties as you wish.
Once you're done, click on Save:
The Upsell offer will appear on your store's cart page for the triggers you selected.