RRAS VPN server for L2TP/IPsec

Hello,

I have a RRAS configured server for PPTP,L2TP with pre shared key on OS windows2008 R2,

I want to make this server as L2TP/IPsec for iOS/OSX client, want to use selfsign certificate rather than pre-shared key, Can you guys help me out what steps require to do this, where to add certificate at server side.

Thanks and Regards,

Jebran.