welcome to Just Answer.
First install the new belt
start engine, have someone inside the car
Stand on the side of the car to see the pulls.
Have the person rev the engine and watch all the pulls and see if any will wobble.
Then have the person inside the vehicle put there foot on the brake and then put the vehicle into drive
Then have the person inside keep there foot on the brake and put it in gear and have them give it gas.
Watch the engine to see if it move forward this is to see if the motor mounts are bad.
If the motor mounts are bad it put to much torque on the belt cause it to break.
The other possibility is if you have A/C and if the belt is breaking during the time when using the a/c the clutch could be locking up causing the belt to break.
Ok so the pulley could be wobbling at high speeds, or the motor mounts are bad, or the a/c compressor. Also if the belt is to small this could be another reason
Please let me know if you have any more questions.
Here is the belt diagram