You will need:
download iphone 3gs iOS 5.0 IPSW
an iPhone 3gs
The ability to say YAY! and follow step by step instructions posted here:
run snowbreeze > feed it iphone 3gs ipsw > Select new or old bootrom accordingly > snowbreeze is about to make you a custom ipsw and asks you for custom settings >Select Expert Mode > general > check 'hactivate' > next > next > build IPSW
Now you have custom iOS 5.0 IPSW! YAY!
Snowbreeze tells you to push some buttons > Your phone is now in pwned DFU mode > close snowbreeze > Open iTunes > click Shift + Restore (to 'restore' your iphone)
Now you've installed custom iOS 5.0 IPSW! YAY!
It's already activated, and jailbroken (tethered only) with Cydia! YAY!
You lost all your stuff so I hope you did a backup of everything! YAY!
Open Cydia > Install Ultrasn0w > restart iPhone > Use either redsn0w or iBooty to boot it up > say "YAY!"
Now you're done! YAY!
If you want to know more about this, you should PROBABLY check out the entire forum section dedicated to jailbreak, unlock, and everything related to that. It contatins tons of information, including everything I showed you here.