1) Check the new belt for length against the old one.
Belts really do not stretch much, they get thinner between the V's and thus go loose.
2)What is he model number from the tag under the seat and also the brand & number of the new belt.
If they have checked out then apply the parking brake to give you as much slack as possible.
Slip the belt over the deck pulley on the engine and leave it between the two pulleys.
Then slip the belt over the fan then over the pulley then off the pulley and on to the spindle
Now go back and work the pulley over the small drive pulley
Go back & run it through the two small clutch pulleys which should be replaced as by now they will be worn out.
On some the belt runs either side of the steering shaft which has to be pulled up out of the way.
On others both side will pass down the same side of the steering stem, just depends who made the mower.
If the belt it tight with the brake applied then either it is routed incorrectly or the belt is too short.