1, www.mongolab.com and create you an account
2, create a instance:
setup your instance by following below picture:
Please use EU location if you are in the EU. It would be faster than us and asia
After you click the create button it will redirect you to the list page, then your instance will be ready in 2 minutes.
Then click your instance and you will see this:
You now have a free mongodb instance, but you don't have an account to connect to it, then you need click the link on above picture, subsequently you will be led to: