Download redsn0w 0.9.9b8 - WIN
Making sure that you do DFU mode correctly is key, it may take a few tries to get your timing right :]
Note that Cydia will not work right after the install, you must boot from tether on redsn0w before it will work properly.
REMEMBER WITH THIS JAILBREAK YOU WILL ALWAYS HAVE TO 'BOOT FROM TETHER' IN redsn0w ANY TIME YOU DEVICE IS RESTARTED OR SHUT DOWN!
Installous if you need it :] Note that after you install installous, you will need to reboot your device again using redsnow, since installous requires a reboot at the end of the download. Use the boot from tether function in redsn0w when you get to your frozen apple screen, you will have to use DFU mode again! To get back to normal, your cydia and installous will be working!