1- First clean to remove any dust, dirt and boot marks.
2- Wipe it down with Paso liquid sander. This will further clean the surface and degloss the door.
3- Find an appropriate color stain. Gel stains work the best.
4- Apply the stain. Let it dry for 24 hours and apply 2 coats of a protective clear.
All of this is similar to <a href="https://www.house-painting-info.com/staining-a-fiberglass-door.html">staining a new fiberglass door</a>.