First, head over to: https://app.datatrics.com/integrations/mailjet/ and click on the "install" button.
After clicking on install, you will see the following screen:
You will have to fill in your Key and App-secret. You can find these in your MailJet back-end. Please see the following article on how to create these keys in MailJet.
After filling in the values and saving, the integration has been completed.