Here is the procedure that I use for these belts. First remove the belt cover, then turn the machine so that it is standing up on it's snout (handles facing straight up) Next remove the belly pan from the bottom of the machine. You should now be able to see straight through the machine. I always start at the bottom and fish the new belt through making sure to get it on the larger auger pulley first. Once the bottom end is taken care of you can set the machine back down on it's wheels to make it easier to work on the top end, When bringing the belt around the top pulley, your idler pulley may case you some issues, if it does then you can just remove the center bolt and take the pulley completely off for the time being. Once your belt is on the top pulley all thats left is to replace the guard and belly pan and you will be up and running again.
You will want to replace the drive belt first with the spring loaded pulley that keeps tension on it.