The following videos help demonstrate the easiest way to fit your Pro-Ject turntable with the included drive belt.
If you've misplaced or broken your drive belt, you can buy a replacement here.
To find the correct drive belt for your specific model, refer to the list here.
For a platter belt (large thin round belt)
Replacement belts can be found here
For a sub-platter belt (short flat belt)
Replacement belts can be found here