How to install custom stable ICS rom on galaxy S2 (rar file)?
I can't see any obvious mention of that particular SGS2 ICS ROM on XDA's SGS2 pages but there are a few other ICS ROMs there, all with install instructions in their main post.

Have you tried opening the RAR file with something like WinRAR or WinZip to see if there are instructions or a readme inside it?