If you're creating a template in the Email Template Editor v1.0, you can make any section editable by putting a special <div> around it.

 <div class="mktEditable" id="UNIQUE_ID">This part is editable</div>
Rules:

  1. The HTML must always be valid.
  2. The class of mktEditable must be included.
  3. The ID must be unique in that HTML.
  4. No spaces in the ID.

mktEditable statements cannot be nested.

If you want to learn how to do this in Email Template Editor v2.0, check out email template syntax.