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 (portrait) you also have the same option. Everywhere you see a Contract name (Open Positions, Orders) you can also hold your finger in the contract name and a menu with options will come up. There you also have Add to Watchlist
To edit the Watchlist, in Quotes Screen you have the Favorites tab. Select it, and in the Watchlist, hold your finger on the Contract you wish to edit. The options to move "up", "down" and "Remove from watchlist" will come up.