Put the belt around the primary clutch first (Smaller one). Get it part way on the secondary clutch (Bigger one) then rotate the secondary clutch moving the belt onto the clutch. It helps alot to support the rear end of the sled so the track is off the ground when you do this so you can rotate the secondary easier.
You can also push in on the secondary clutch and rotate it counter-clockwise so the sheaves open and part. The belt will drop onto the secondary. I personally find the first method easier. I've heard of guys wedging a piece of wood between the sheaves of the secondary to separate them as well.
Good luck.

What model is your Yamaha 700?. I have a 99 and 2000 Mountain Max 700.