Perform a Vetcove search that includes the item you'd like to add to your List.  

In an individual product listing for the item, locate and click on the Lists button.

Your existing Lists will appear in a tab that drops down from the product listing under the heading Shopping Lists. Click the existing List you'd like to add the item to. The gray circle with the "+" icon will turn into a green check mark to indicate that the item is in the List.

First, find the List for the product. This is most commonly done when viewing the items in an existing List, but can also be done by searching for the product.

Click the List button on the item. If the item is in a List, there will be a green button with a white check mark next to the List(s) the item is in.

Click the List you'd like to remove the item from. The check mark will now turn into a "+" and your item was removed from the List.