Open Cydia, tap the Manage tab, tap Sources, then the Edit button, and finally the Add button. You'll get a window that starts you off with http:// to which you'll want to add cydia.iblogeek.com. (Note that there's only a single "g" in the URL.) Once you've got that in there, go ahead and add the new repository. Cydia will do its thing and reload itself, then ask you to return to Cydia.

Tap the Search tab in Cydia and search for iUsers. When the results load, select iUsers and install it. When it has finished installing you'll be asked to restart Springboard (your home screen app), which you should go ahead and do.

After Springboard restarts the lock screen will come up and you'll be asked to set up multiple users before you can use it. Go ahead and set up your first user by following the on-screen prompts.

When you're done, go into the Settings app on your home screen and look for the new iUsers tab. It should be about halfway down. Tap it and you'll be able to add additional users. This is where you can create a guest account.

Once you've created all the users you want, lock your phone and try to unlock it. You'll see a little icon with people in it inside of the "slide to unlock" bar. Tap that and you'll be able to choose the account you want to use. (Note that on the iPhone and iPod touch there's a bit of a glitch that makes it hard to see the account names, but you can still select them. Watch the video above for help with this.)

That's all there is to it. Just perform the same user selection steps described above to switch back to your other account. Now you can have multiple users on your device, or just have a guest account when you don't want your friends getting into your data.