Before your emails will start automatically showing against contacts, you need to start sending them to your "Unique BCC Email Address". There are two methods to achieve this:

When sending an email to a client, you can simply BCC the email to your unique BCC email address.

Many email programs allow you to create rules that automatically perform actions on your emails. You can use this functionality to automatically BCC every email you send to your unique BCC email address. Below, we have compiled tutorials (written by third parties) for some of the major email programs: