i am doing this as we speak. you have to work yourself backwards. start at the axle: where is your sprocket?
then run the chain from axle sprocket to a sprocket on the jackshaft of the secondary clutch.
then on the secondary clutch run your sled belt to the primary clutch on sled motor and align it. your motor should be about 12.25" from secondary. (that's center of primary to center of secondary)
if you have no idea what the terms are i just used, please use the search function on this site and others. snowmobileforum.com is awesome for info and manuals. pm me for more.
Its different for every sled. There are hundreds of different belts. You need to get the center to center distance from the original sled, or get a belt and estimate a good distance.