You need to link and initialize the font addon, then use al_grab_font_from_bitmap()
to load a bitmap to a font. Look at the documentation of this function to learn about the required format.

You could make the font bitmap with any image editor or image tool you like, as long as the format is correct. Basically, the background (transparent) color is at the top left hand corner ((0,0) coordinates) of the bitmap, and every glyph needs a rectangular border of that color around it. Look at this thread for a few example bitmap fonts that can be used together with Allegro:

https://www.allegro.cc/forums/thread/610576