Well we first have to know some stuff...
Do you have photoshop?
Do you have all 60 plugins for HMT3.5?
Do you have the .dds plugins that you need for photoshop so you can see the textures for halo because the textures in halo are in .dds format?
If you said YES to any of those..then your ready to start creating some textures for HALO!
What you can do is open up the bitmap tab in HMT3.5 for your .map file. You'll get a big list of textures. When you click on one, most will give you a preview of what the texture looks like and a little something beside it. It might say DXT1 or DXT2/3 or DXT4/5. Remember that. Press "save texture " in HMT3.5 when you selected your texture.
Now fire up photoshop and open up that texture that you saved. Start modifying it or whatever
(make sure you have a backup of the original texture file just incase something goes wrong) then, if you got different layers, go to the layer button on the top tool bar in photoshop and select the "flatten" option. Now save it as a .dds file. If the file that you saved said "DXT1" in HMT3.5 next to the little preview texture, press these options into the Nvidia Tools window that popped up after you press saved as ex Assault rifle_fps.dds
DXT1 ARGB (1 Bit Alpha)
No Mipmaps
2D Texture
Then press save. Now go back into HMT3.5 and navigate to your bitmap name ex,
Weapons\assault rifle\fp\bitmaps\assault rifle_fp
and press the inject button under the texture preview window
select your "texturename".dds file then press yes/whatever/ok
Now your texture is in the game Congrats! If anything goes wrong or looks funky just re-inject the original bitmap. I always make two copies of a texture. One that is the original (just incase I need it if things screw up) and one that is the modded one)
GOOD LUCK!
PS: If the texture you want to modify says anything other than DXT1 then try these options for the Nvidia Tools
*{If it is DXT1, then click the DXT1 (1 bit Alpha). If it is DXT2/3, Select DXT3. And DXT4/5 As DXT5 and if it is A8R8G8B8 select DXT1}*.
These are the settings I use when saving a file as a .dds A8R8G8B8 File:
8:8:8:8 ARGB (32 bit) Generate MIP maps (all)
2D Texture
(not sure if it was use existing mip maps or generate...just try both..sry can't remember that part

)
PS: If you see some textures that have something with multipurpose in the m then those are files that define where on the texture is light allowed to shine. All that purple stuff means that over that area of the texture it is allowed to shine. All the blue stuff is still shine just more generic..red means really shiny and green means glow 8D
The more vivid/bright the color is the shinier it is. Black means no shine, 100% Opaque purple (FF00FF) means REALLY SHINY.
You can automatically make your own by opening up the original texture file of the multipurpose bitmap and duplicating the background layer because you can't edit the background layer, then go to Layer>New Fill Layer> Solid Color then in the solid color window, select the mode to color (at the bottom). Then set the color as
FF00FF
in the
#:
area
..
so yea.
All the best luck!