I have a 8yr old craftsman riding mower with a 42in deck I went to get a belt and I got the right one but I can not get it to go on do I need to remove any of the pullys or are there any tricks that I need to know or should it just go right on it is about 3inches to small. And I did disengage the blades but still will not fit

Yes disengage the blades and look for a pulley that is not directly above a blade. This would be your tensioner pulley and it is spring loaded to apply tension to the belt. It is moveable and should give you the room needed for the belt.