1: install horizon ( also made by the wemod community )
2 : go to this link and pick the dlc you want to download
3 : after downloading the file extract it ( i personally use winrar )
4 : after extracting the file open horizon and drag it onto the screen then save it into the flashdrive
5 : remove the flashdrive from the pc and put into the xbox and then thats it enjoy
6 BONUS : if you want the dlc to be in the xbox hardrive itself then go to the main menu then go to storage and simply either copy or move the file from the usb to the xbox hardrive
some answers for common problems
q : why doesnt horizon want to read my flashdrive
a : because you didnt configure the usb from the xbox 360 itself
q : when i start the game it crashes why ?
a : you probably dont have the latest title update just go this link http://xboxunity.net/