So, once it's decided what work to do, it's just a matter of resuming all the fibers in order.
Additionally, since fibers know what work to do next in some cases, the overhead can be very small. You sometimes don't need to yield back to the scheduler, but can resume directly another task.
That being said, a good scheduler is basically just a loop, like:
https://github.com/kurocha/async/blob/bee8e8b95d23c6c0cfb319...
So, once it's decided what work to do, it's just a matter of resuming all the fibers in order.
Additionally, since fibers know what work to do next in some cases, the overhead can be very small. You sometimes don't need to yield back to the scheduler, but can resume directly another task.