(1) Make sure your device is jailbroken, and you have an app called Cydia on your device. If you're unsure, check out this link:
Jailbreaking
(2) Go into Cydia. Wait until it loads a screen similar to this:
(3) Tap on the Manage tab, along the bottom bar (second from right), so you get a menu like this:
(4) Tap on the Sources tab (middle option), so you get a list of sources:
(5) Tap on Edit in the top right corner, so it displays this:
(6) Tap on Add in the top left corner, so you get this:
(7) Type in cydia.hackulo.us (NOT .com or .org). Click Add Source and wait for it to load, then tap Return to Cydia.
(8) When it finishes, you should have a new "Hackulous" Source. Tap on it, so it displays a list of packages, then scroll down to Installous 4, and tap on it.
(9) Tap Install in the top right:
(10) Tap Confirm in the top right:
(11) After Cydia finishes installing the packages and "Reloading Data", tap Return, exit out of Cydia and Reboot (turn you device OFF, then ON again). You should now have an Installous icon (somewhere) on your homescreen. Enjoy, and use Installous wisely
