iOS app:
There are two ways to do this. 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.
Android app:
To mark an item as complete, press on the recipe, workout or article and press on the tick icon in the top right corner.