Whenever you are editing a To Do item you can mark it finished by checking the box in the bottom right corner of the editor:

Another easy way to mark the item as finished is by clicking on the check box next to the item and then clicking on the "Finish" button to mark the item as finished.

Finished items do not appear when you are viewing either your Current or Future items. To see your finished items, choose Finished from the status drop down menu in the show bar:

Your finished to do items are displayed with satisfying orange check marks next to them and stamped with the date they were marked as finished:

You can always change the status of a finished item to be unfinished by editing the item and unchecking its "Finished" check box.