I can be contacted at email@example.com if these steps don't answer your questions feel free to email me.
1) Code an XHTML standard compliant email with the parent most table being a width of 600px
2) Inline your styles!! If you didn't know already Outlook isn't a huge fan of styles not being inlined and may cause adverse styles if you don't inline.
- Using the above referenced address copy and paste into File Explorer (This process only works with Windows btw).
- Copy and past the .html file you've just created using steps 1 and 2 into this file (aka. the Outlook Stationery File).
4) Once you've done that simply start outlook and up next to new email there's an option for :
new items -----> stationary ------> Find your file in the list of stationary
5) Once the file is opened in Outlook simply choose save as and change the file type to .oft
I have a lot of experience creating these and distributing them to my company (CHG Healthcare) let me know if you're having any questions