We are just developing this as a service to our clients, so hopefully I can help.
One of the ways is to...
Open Outlook Express / new mail.
In the "new Mail" window..
Go to Format and change from plain text to Rich text (html).
From there you are in an "generator".
You can select source code and add style sheets/ tables / images / anything else.
I tend to cut and paste the source code to note pad and "save as" as a backup.
Once you have finished editing the page, you have two options.
a) save in note pad and when you want to send, cut and paste into the "New Mail" as above.
b) "Save as Stationary" .
When you are ready to send, in the main Outlook page go to "tools / Options / compose " Under "stationary" click the box then "select". You will get a box with all the standard stationary, plus the file you created. Obvious from there.
Restart Outlook, select "new mail" and the page should open with the new "stationary" you selected.
This is how I have done it, there may be a better way?!?!
Hope this makes sense and helps.