Usually there are two belts. One goes around two pulleys, a front pulley under the motor and one on the mower deck. Additionally, there is usually a spring tensioner pulley that rides against the belt to allow it to work and for ease of routing.
The second belt goes under the first belt and is routed on a common pulley with the drive belt and both (or all three) of the mower blade pulleys.
Here is a crazy looking exploded view of a craftsman mower deck
And here is a better diagram of it all put together. I think you can get an idea of belt routing out of this one: