I'll try to explain to you first:
Hold the analog up, and tap down twice with the dpad while holding the analog.
Then slide your analog in this motion:



When you get to the

tap



on the dpad. And press B+G. You won't get iCS on your first try. It's just a bit of practice, and don't worry, it really doesn't take long to get used to.
iCS is obviously easier online, however I have got it offline with this method, my thumbs get quite sore trying to do it offline :P When you think your consistent with CS, try going online with a friend and practice is on him/with him. Try doing what you have been doing so far, just a bit faster.