Hi Sang
Do you want a letterhead or an invoice, or both?
If you want to add a letterhead to your invoice, that's easy. Decide what
you want, and add it above the detail rows of the invoice. If you want to
include a logo, save the logo as a graphic file, and insert it with Insert,
Picture, From file.
Other than that, you need to be a lot more specific on what you wish to
achieve, before anyone can really be of assistance