There are two ways that you can mark an item as complete.
To mark an item as complete simply swipe the item you wish to complete to the right of the screen. Once you do this you will see a tick appear, press on the tick. Alternatively you can swipe the item the whole way across the screen and it will be ticked as complete.
To mark an item as complete, press on the recipe, workout or article and press on the tick icon in the top right corner.