However, those letterheads are often provided in some [proprietary format](https://en.wikipedia.org/wiki/Office_Open_XML) with which I do my best to make as few interactions as possible.
Thus, thanks to [Nicolas Trotignon](https://perso.ens-lyon.fr/nicolas.trotignon/) who found a simple workaround, here is a small trick to do it.
As a preliminary, you have to obtain a pdf version of the letterhead (to do so, I'm using [online office 365](https://en.wikipedia.org/wiki/Office_365) as it works in Linux, yes, shame on me).
Once this is done, the core of this method is to use the [wallpaper](https://www.ctan.org/pkg/wallpaper) package to embed the letterhead as a background image, and then play with margins to make it look nice.
Here follows a minimal example with the [corresponding output]({static}/examples/background-latex.pdf). Replace `background.pdf` with your (empty) letterhead of course.