This jailbreak needs Limera1n software that you can download from the official website. Blackra1n for OS X / Windows doesn’t install Cydia, it installs an app that lets you download Cydia. It’s recommended to use Wi-Fi or 3G to install Cydia after the jailbreak.
Requirements
System requirements: iOS 3.1.2
Compatible devices: iPhone, iPod Touch, iPad (all generations)
Guide
1. Plug in your device using USB and close iTunes.
2. Download and run Blackra1n.
3. Click the only button there is on the screen of the program.
4. It will make your device enter Recovery mode and jailbreak it.
5. If the everything went ok you’ll see this message:
6. Now you need to connect to Wi-Fi, preferably with fast connection. After that run the newly installed Blackra1n app on your device.
7. Select Cydia installation in the app
8. The app will start to download and then extract Cydia
9. After Cydia is installed reboot your device. Then open Cydia with Wi-Fi on and give it a few minutes to update system files.
10. Your device is jailbroken. There’s no need for Blackra1n anymore and you can freely delete it by tapping Uninstall Blackra1n.
11. To have a free access to the file system through WinSCP, iTools or i-FunBox and other file browsers you need to install afc2add package from Cydia. To do that:
Go to Cydia > Search.
Type in afc2add into the search bar and find the package you need. Tap Install and reboot the device after the installation.