First of all, find a skin you like. there are some good skins here http://files.halomods.com/viewtopic.php?t=52302
Now, download HMT 3.5 from the Utilities forum and, here is think link: http://files.halomods.com/viewtopic.php?t=4198
This is a great program made by monoxide and used by lots of modders. it does the simple stuff like change the damages of all weapons, falling damage , weapon speed and projectile speed. now, back to skinning.
First, if you have windows press the windows key and select search, put in the file 'Halo' in the search box and after you see the SECOND halo folder, click it. Then go to halo > maps and copy the bloodgulch.map file and place it on the desktop screen. move the REAL bitmaps there and then open HMT.
You will search desktop and find blood gulch, open it with HMT.
There is a huge list of stuff. just earch around for bitmaps, it should be near the top.
now, if you have found it lets say you have a file called cyborg1.dds.1.dds
Just click on the file in HMT that ends with cyborg. press the inject button and find the skin you want to place in (cyborg1.dds.1.dds) once you have done that, usually all skins come with another cyborg file.. this is for the hands. lets say it says cyborg hands1.dds.dds. go to HMT > bitmaps again and find cyborg hands. inject the 'cyborg hands1.dds.dds' file into the normal one. you can now delete the copied bloodgulch on your Desktop and DONT delete the bitmaps file. Bring the bitmaps.map file back to the maps folder and your done