How do I play a daily challenge?

Depends on the challenge. Check the events tab for the current daily challenge (they run for 55 minutes with a 5 minute break) each. It will tell you what the requirements are to earn points. You can check our Daily challenges schedule page for a list of what to expect each day.

Complete the tasks during the time for the event to get points, if its building upgrade for example then get the upgrade to finish at the right time.