Use Krayzie 1.1.1 gamesave plus Splinter Cell / Mechassault (original release only) / 007: Agent Under Fire (original release only). Krayzie is the best, most newbie friendly softmod, and it comes with instructions. You have a choice, but install UnleashX - it's newer and better than Evox as a general purpose dash.
You need to softmod the Xbox before you can use AID (or any other homebrew disc), but don't put AID anywhere near your Xbox until you know what you are doing.
You can install additional dashboards (i.e. XBMC) and emulators via FTP or from DVD.