One way to mark a Task as complete is to select the Mark Complete in a List View or Tree View for the Task you want to mark complete. When you click the Mark Complete a prompt will appear asking for closing notes.
In the List View, you can mark a task (or multiple tasks) complete by selecting the checkboxes for all of the Tasks you want to mark as complete. Then then click the 'Mark Complete' button at the bottom of the List View. A prompt will appear asking for closing notes. These closing notes will be added to all the Tasks that were selected. All these Tasks will have their % Complete value modified to the value of '100%'.