Mark as completed add-on lets you configure any requirement to being marked as completely manually by the user. You can find a step-by-step guide here.
There are some achievements using this add-on features, just mark as completed each requirement to see how it works.
Example using checkboxes:

Mark as completed (checkboxes)
You can unlock this quest by manually marking each requirement as completed. Used for the Mark As Completed demo.
3 Steps
- Do 100 push-ups
- Do 100 squats
- Do 100 sit-ups
Example using buttons:

Mark as completed (buttons)
You can unlock this quest by manually marking each requirement as completed. Used for the Mark As Completed demo.
3 Steps
- Test AutomatorWP
- Visit our Amazon’s account
- Review GamiPress on wp.org
Example using different style settings and some other requirements:

Mark as completed (multiples tasks)
You can unlock this quest by manually marking some requirement as completed but also there are requirements that needs other actions to perform. Used for the Mark As Completed demo.
8 Steps
- Mark as completed (Checkbox before)
- Mark as completed (Checkbox after)
- Mark as completed (Button before)
- Mark as completed (Button after)
- Mark as completed (Custom label button)
- Visit the Mark As Completed demo page
- Unlock “Mark as completed (checkboxes)”
- Unlock “Mark as completed (buttons)”