To Add/Remove items from your Watchlist you first need to be logged in. If you don't have an Account, you can create a Free Demo Account
To add a contract to your watchlist, go to the contract (find it in search, browse for it...) and in the Contract screen you will see a "+" menu in the corner with options. One of the options is "Add to Watchlist". Inside the Chart screen you also have the same option.
To edit the Watchlist, in Quotes Screen you have the Favorites tab. Select it, and you will see the Edit button beside it. Click on it, and you will be able to delete items and reorder them.