Im pretty n00b here to the xbox scene but i have some experience i think i can share with you.
If you have never soldered b4, the soldering will probobly be very intimidating. dont worry though, though they seem small the pins (and rebuild board) are relatively easy to solder. Make sure u have some desolder braid just incase you bridge a connection. There are tutorials here that tell u how to do it.
after that you will have to get a bios for your chip (the x3 bios is recommended). Because most bioses are illegal i cannot tell u exactly where to frind them. Customize your bios to your liking.
If using the x3 bios use the automatically partition harddrive option. This will setup your required xbox partitions for you.
You need to find a dashboard now (if you are not running linux). Find it the same way you found the bios ^_~.*(xbins, cough)* If running the x3 bios its easy, extract the contents of your dashboard strait to your C: partition. If the .xbe file is called default.xbe rename it to one of the autostartup dashboard names in the x3 bios ( eg. avalaunch.xbe, evox.xbe, xbmc.xbe etc). People say Evox is the best for newbs, i would say avalaunch is pretty kool to start off with too (thats what i started with).
Try all the dashboards, all you have to do to run another one is use the file browser in ur current dashboard and run the xbe file for another dashboard.
Remember that if the dashboard u are using doesnt suit you, you can always edit the .xml files included to your liking.
good luck and happy modding