Lay the vacuum cleaner flat so you can access the bottom plate.

Using a medium Phillips screwdriver, remove the five screws holding the black bottom plate and lift the plate off.

Remove the small side plate for access to the belt attachment points.

Remove the broken belt and stretch the new belt from the small drive shaft on the motor to the wheel on the roller brush.

Here's what the belt should look like when properly attached.

Replace the small side cover and attach the bottom plate using the five screws removed in the disassembly step. This would be a good time to clean out the bag, too. That's it!