This is my first post here, so I apologise if this is in the wrong place, or whatever, and it's probably not a very well designed guide but it gets the point acrossThe screenshots weren't the best positions or angles so do your best to make them out :P Theres other ways of getting to C'Thun thats been posted but they all require a Chopper or multi-seater mammoth, this way is cheaper.
First you enter AQ40 and you clear the trash up to the first boss till you're stood in front of him
http://i752.photobucket.com/albums/x...010_190606.jpg
Then, you stand in the position in this picture
http://i752.photobucket.com/albums/x...010_190625.jpg
Then, you mount up on some kind of mammoth, and it'll look like this
http://i752.photobucket.com/albums/x...010_190634.jpg
You jump through the wall, and you'll land here
http://i752.photobucket.com/albums/x...010_190652.jpg
Jump down so you're on the same level as the big hole in the floor, and follow the path round to the far side of the hole furthest away from you
http://i752.photobucket.com/albums/x...010_190700.jpg
http://i752.photobucket.com/albums/x...010_190723.jpg
when you reach the far side, you should stand in a position which looks like this
http://i752.photobucket.com/albums/x...010_190741.jpg
when you reach there, just jump forward into the hole, you'll end up in the corridor just before C'Thun's room. Enjoy!