I will gladly help but you really need to be more specific, I suggest you go to youtube and then search for a walkthrough of that chapter if you really are stuck, altough it really isn't hard or something, you just missed a handhold somewhere or something, just have another look, good luck!
I had a hard time with this too...when you jump off the first ledge onto the rail on the wall, climb down the wall until you're at a kind of window thing. Go through the window so you're at the other side of the wall. Keep moving to the left until you can't move left anymore (there should be some debris under you..Then simply drop down.
Hope this helped.