How do I check in that I've completed an item?
There are a couple of ways you can check in, depending on the device you're using. On any device or the web, you can tap/click the habit name to go to the detail screen. On that screen, tap/click the large checkmark icon; it will change color and then give you an option for leaving a note about your check-in.
On Android, you can also tap the checkmark that appears on your dashboard to check in without tapping through to the detail screen.
On iOS, you can swipe to the right on your dashboard to check in. You can also use the widget in your notification center (if you've enabled it) to check in without opening the app.
Did you miss a check-in from the past? To add a check-in yourself for any past date, start at your dashboard. Click/tap on the habit for, then on "weekly stats". Choose the Calendar tab. Then just click on the day you want to add the check-in for. A screen will appear to let you add the check-in and any notes.