Base will automatically attach email correspondence with a contact onto any open deals associated with this contact as part of the email process.
If you find that emails are not attaching themselves to related deals this may be because the deal has been moved to a 'Closed' stage. Once deals have been moved to a 'Closed' stage Base no longer adds email correspondence to the deal. If you would like email correspondence to be displayed on the deal you will need to move it to an 'Open' deal stage.
You can also associate emails with Deals from the Communication Center. This option will appear automatically when you hover your mouse over emails in your Inbox or Sent folders.