Here.. http://wiki.e3d-online.com/wiki/BigBox_Printer_main_assemblies:_Step_By_Step_Assembly_Process Just following the note on what length to cut the belt (690mm) it shows the belt being fitted to the X-Axis Idler. (It does not indicate front or back of the Idler??) It is suggested the belt is upside down in the photo (with teeth downwards?) but to engage the teeth with the pulley on the rear bar it needs to have the teeth facing downwards which is the way the belt is in the picture "around the pulley" The belt fits teeth upwards in the X-Axis Idler (if you look through the slots in the side panel you see teeth facing down to where it should locate, being held with a grub screw) But if the belt is fitted teeth teeth up then the belt would have to be twisted 180o to engage on the pulley on the rear bar. Pictures like the one with comment... "Continue to feed the free end of the belt through the X-Axis Idler. " are not much help as it's impossible to see anything from the photo.