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 Star in the bottom menu. Clicking on it will "Add to Watchlist".
To edit the Watchlist, fist slide to the Watchlist section. You will see the Edit icon in the bottom menu. Click on it, and you will be able to delete items and move them by using the cross, left and right arrows that will appear on each contract. When you are done, click the Edit pencil icon again.