To check-off items, simply click the checkbox to the left of the task you've completed.
Upon doing so, you will see an option to Hide Completed at the bottom of the page, along with a number in parentheses indicating the number of checked-off items you have in your project. Click the hide button to hide your completed tasks.
As you check-off items in your project, you will notice a progress bar keeping track of all the progress you've made with your project! Time to GTD 💪