When you go into guerilla go to the assault rifle's folder and open assault rifle.weapon_hud_interface. Once open scroll down some and you will find tags like static elements, meter elements, and crossairs. Static elements and meter elements will house the bullet amount, or heat amount. Crossairs is self explanatory. Then at the top says:
child hud [ui\hud\master rounds] [...] [open]

(if its a plasma weapon it will say:
child hud [ui\hud\master plasma] [...] [open])

Open that up and go to the static elements. Now this houses the main hud part of all weapons. If you have a new one you will want to place the path to the new one and save the master rounds as something different like master rounds new. This goes the same with the assault rifle.weapon_hud_interface. Save that as something different. That way if you dont like the way you have changed it, you will still have the same old one.

Hope this helps.

O ok, i missunderstood. My bad. You can go here: http://ghost.halomaps.org/tutorials/ to get a tutorial on make a custom hud as well as other tutorials for making maps. Its a greate source.