Below are links to pdfs that will help you begin to design your character letterheads. You should open “Logo Elements” and “Letterhead Template” in Adobe Illustrator. You can open the Dummy text in Preview or Adobe Acrobat.
Logo Elements – How to display your logo, typeface choices, and color scheme.
Letterhead Template – Letterhead layout with guides. Change to fit your design concepts.
Letterhead Dummy Text – You won’t know how successful your letterhead design is until you’ve dropped in some content!
Here’s a link to The Designers Toolbox – #10 envelope dimensions and downloadable .eps file
Number 10 Envelope Help sheet – dimensions and design considerations