To create a Checklist you must first create a Task, if you haven't check out this article.
1. Once you have save your task navigate over to the Checklist tab by clicking on it.
2. Click the + New Checklist link down the bottom of the window. Give your Checklist a name by typing in the box and hit enter.
3. From here click the add link and type an item for the checklist. Click Save.
4. Continue on clicking the add link until you have all your items added to your checklist.
5. To check off items on your Checklist, just tick the check box. And it will show you your progress above via a percentage.