When you've successfully rebooted, Cydia will be on one your pages of apps, usually not the first page.

Launch Cydia and it will reorganise some files and then respring your iPod, which is like logging out and then back in to your PC almost.

Launch Cydia again and it'll ask whether you're a User, a Developer or a Hacker. Most people are users, leave the scary stuff alone, you probably won't need it.

Cydia will now refresh it's sources and reload its data, once that's done, you can browse it's vast collection of tweaks, apps and themes.

You're done! If you have any problems, check out jailbreakqa.com which is full of helpful people who know all about jailbreaking.

Credit to @comex for the exploit, untether and jailbreakme.com and huge credit to @Saurik for writing, developing and maintaining Cydia.