How to Create "Stationery" for your emails, using Gmail and Thunderbird

Gmail account settings required
  1. Assuming you're sending these emails from a gmail account, you'll need to configure Gmail to enable IMAP access.
    (IMAP is what Thunderbird uses to communicate with the Gmail servers.)
  2. In Gmail, click the gear in the upper right corner and go to "Mail Settings"
  3. In Mail Settings, click the "Forwarding and POP/IMAP" section.
  4. In IMAP settings, make sure the "enable IMAP" box is checked.
  5. Click the "Save Changes" button at the bottom.
Software Required
  1. First, install Thunderbird. Get it here -- http://www.mozilla.org/en-US/thunderbird/
  2. Once Thunderbird is installed, open it up.
  3. From the Tools menu, select Add-ons.
  4. In the "Get Add-ons" section, go to the "Search all add-ons" box in the upper right, and search for "stationery".
  5. Stationery version 0.7.6 should show up in the results. Click the "install" button on the right of it.
  6. Once the add-on is installed, Quit and Re-open Thunderbird.
  7. Next to the "Write" button there should now be a little dropdown arrow which lets you specify a type of stationery.
Configuring Thunderbird
  1. First we need to setup Thunderbird to be able to send/receive email using the Gmail servers.
  2. Fortunately, there's an entire page here which explains how to do it -- https://mail.google.com/support/bin/answer.py?hl=en&answer=180189
  3. Please note that while those instructions are written for Thunderbird 3.0 they should still work roughly the same for whatever the current version of Thunderbird is.
Creating Stationery(This is probably the trickiest bit)
  1. In Thunderbird, click the Write button to compose a new email.
  2. Click inside the main composition window.
  3. Using the "Format" option on the main menu bar, change how the email looks to be a template for your emails. (Background color, font color, background image, standard tables, signatures, links, etc.)  Basically everything except that actual text that will change with every email.
  4. Once it looks how you want the stationery to look, click the dropdown arrow next to the "Save" button and save it as an HTML file.
  5. Close the Write window once you've saved your stationery template.
  6. From the main window of Thunderbird, go to the "Tools" menu and select "Add-ons"
  7. In the Extensions section, you should see the Stationery add-on listed. Click the "Preferences" button to the right of it.
  8. In the Stationery options window which pops up, you should be looking at the Manage Templates section.
  9. Click the "add new" button and browse to the HTML file we just saved.
  10. The template should be added to the list.
  11. Close the Stationery Options window.
Assuming everything went okay up to this point, you should now be able to click the dropdown arrow next to the "Write" button in Thunderbird, and select to compose a new email using the stationery you just created.
Simple! ;)