How to install ClockWorkMod recovery on a Samsung Galaxy S2 plus gt-i9105?
First: Cyanogenmod is a customized version of Android and NOT a recovery.

Ok, let's move. 
You should root your device first. Once rooted you can install the ROM Manager App from the Play Store, which will guide you to install Custom Recovery.
In order to root your S2 follow this instructions: http://forum.xda-developers.com/showthread.php?t=2663834

You also can follow this tutorial: http://youtu.be/xBJyQrbEkuc