how to restore my app data backup from iTunes to iPhone

I have application Smurf Village and now I'm on level 27...

And I have backed it up 2 weeks ago to my iTunes...

suddenly today the apps crashed and when I restarted my iPhone, everything's gone...

So i have to start over from level 1...

How to restore backup from my iTunes (that got the apps backup) to my iPhone ?

Since if we plug in our iPhone to iTUnes it will sync automatically....(my backup will also back to level 1) ...

is there any mechanism or step by step to restore it ?

please kindly advise

regards,

Dani...