Ok ;o)
First you have to download and install a MOD Pack (I'm playing with HSFX 4.11) Ultrapack is not bad.
When it will be finished, you will just have to download and install Mods that you want to play with (new planes, new effects...)
Don't forget, to test your game at each step, and to BACK UP your files ;o)
Many times I didn't back up my files, so I had to reinstall...it was frustrating...
What I suggest, follow Newbie guide, when you will have installed the MOD pack that you want, test it and back up your UBISOFT FILE (I'm using an external drive to do it).
After that you will have to search on this site which mod you want to play...
Here is another link :
http://www.sas1946.com/main/index.php/topic,97.0.html
This is SAS community, also working on MODDED IL2.
Welcome in Modded game ;o)
Ted