the basic method is..
Run halo 2 or later live enabled game and tell it to connect to xbox live (you dont need to be online or connected to a network or anything)
you do this to update your dash.
then you find a way to transfer the game saves to your xbox (there are pal and ntsc game saves)
You can do this with USB adapters, controller / cable mod, or action replay excetera. not all usb drives will work, so i use sd cards and sd card readers, the general rule according to the community is that the older the usb drive the better.
Alternetavly ytou can hotswap the files on to your xbox..
to format the usb drive you only really need to connect it to your uusb adapter then go to the MS dash's memory manager. it should detect and format the drive,
You then use xplorer360 on your pc to transfer the files to the usb
Then you take teh usb back to the xbox and transfer the saves over
(remember you have to press right to highlight the actual save, what you initaly highlight is technically just a folder)
once the save files are on there you just use splinter cell to load the exploited saves. (a note about this, Its customary to only have the exploitable saves on your xbox, and not any other splinter cell saves "please dont defy convention")
once you have installed the soft mod via splinter cell its on there and its not coming off unless you remove it...
One final step i would recomend is to download hexen utility disk and burn it to dvd at 4x speed or slower (i usualy forget to mention the speed)
Then boot your xbox with that disk installed, it will update your softmod for you and its a good disk.
finaly set up your network so you can FTP the eeprom.bin file to your pc. This file will eventually be needed so dont skip that part.
and thats the basics of it all..
If you need a step by step tutorial there are plenty of them here
The usual places are xbins excetera,
but google can find you files like..
Krayzie Splinter Cell Ndure Installer
(do you need the pal or ntsc)
heimdalls HeXEn xbox engineering disc