In order to link to a previous email, you'll need to first find the "permalink", or permanent URL to that previous email.
To do this, go to "Reports->Email Tracking". Select the previous email you'd like to link to and open the report.
You'll see an item on the Summary screen called "Permalink". Copy that link to your clipboard or to notepad for future reference.
You now have a link to the previous email, and you would insert that link like any other link. Noted here for reference: Creating Links