Simple thing!! Just put the letterhead in your printer feeder and print the information using the printer. The content will be printed in your letterhead.

Couple of things to be considered:
- Make sure to have some margin at the top of the page to avoid writing over the original letterhead content.
- Make sure you place the letterhead correctly in the printer feeder so that when you print in your letterhead it does not print on the other side of the paper.

To avoid wasting your precious letterhead(s), try printing on a regular paper as suggested by Kiddo and compare it with your original letterhead to find out where the content will be finally placed.
Also make sure that you select A4 for paper size. Finally take the print-out and someone can physically sign it.