its similar to jailbreak.. and download the software.. either mac or pc.. just click on the button and u'll be done..

the process is similar to blackra1n but blackra1n for 3.1.3 isn't out yet.. they have the jb and unlock but are waiting for the release of software 4.0 to be out... 

also i think its a good idea to get ur shsh before you jailbreak so incase u accidentally updated ur os then u can use it to restore ur previous version of the os..

 

@ bhatke dai.. its necessaryevil with 3 replacing the e.. and also i think 3g with 5.12 baseband can't be downgraded..