HowTo Jailbreak Firmware iOS 4.2.1 for iPhone 4

Redmondpie.com's has this very straightforward guide (Jailbreak iPhone 4 4.2.1, iPad, iPod touch with Redsn0w [VIDEO GUIDE] | Redmond Pie) to jailbreak your iOS device. According to the video, this instruction is suitable for 4, 3GS, 3G, iPad, and iPod touch. Written instruction is here.
You will need:

1. iOS 4.2.1, which you can find the links here;
2. iTunes 10.1 for Windows or Mac;
3. Redsn0w 0.9.6b4 for Windows or Mac (if the link the zip file is broken, just google "Redsn0w 0.9.6b4", I found a link here);

First install Redsn0w, if you're using Windows, first scan the zip file with an anti-virus program (you should too if you use a Mac, and don't get me started on this), I use Malwarebytes' Anti-Malware 1.46;
The video and written instructions are using Mac, but the Windows screen is almost the same, so just follow the instructions. One note though, this jailbreak is NOT a untethered jailbreak, which mean you will need to boot it to the "jailbroken state" and connected to a computer every time you reboot your device. I don't reboot my iPhone very often, but need to keep in mind though, the jailbroken iPhone is NOT as stable. I'll update the blog once the "untethered" jailbreak is available.