Just as the subject says. How do you make a labelled minimap? I tried searching for this answer but didn't have much luck outside of making a regular minimap.

After "buildminimap" the entire process is manual I'm afraid. Most Half-Life sprite tools allow you to save the sprite into its component images in bitmap form, I use Sprite Viewer (link).

I personally put all of the bitmaps into one big bitmap to make them easier to work with. And from there I manually clean them up, color the vents, and then label them. I then break the big bitmap down into the little bitmaps again and run a sprite generator program to make the new sprite. I actually use the command-line tool that was released with the SDK called "sprgen", but there are several GUI programs that can create them as well (link).