Step 1 : Get your iPhone jailbroken.
Step 2 : Add source to cydia
Step 3 : Install Siriport
Step 4 : Goto Settings > SiriPort.Ru Original Dev Pre > Install Certificate > Reboot your iPhone
Step 5 : Goto Settings > Genearal > Siri > Enable Siri
Step 6 : Reboot your iPhone and Make sure you have internet connection.
Step 7 : Down hold your home button and try to say 'Hi' to siri and waiting until Siri is activated,
You may try couple times and waiting couple minutes.
Step 8 : You can ask Siri anything you're doubt with