Also, keep in mind that there is more that just the shoppers to consider when scheduling. MSCs with an editing staff need to have the work parceled out evenly over the course of the month so the shops can be completed in a timely manner. That is why many don't allow self-scheduling.
A scheduler may have many great shoppers to choose from on a particular shop, but consider that shoppers often apply for multiple assignments with the same company for the same day. The scheduler then has to consider the rotation and assign the best candidate for the optimal editorial window, while trying to assign the hardest to schedule shops first, then following up with the easier ones. It's like solving a giant puzzle if you have 250 shops that get posted on the 1st of the month.
When I was scheduling, I would have a giant poster board at the beginning of each month with the calendar on it. I had cards with each of the shoppers names on them and would watch the requests come in online and start assembling the calendar. When I had it flowing well, I would go in and start assigning, but often that took about 3 days!
I'm sure each scheduler has their own method, but just know that it can be a difficult and time-consuming process. I had to make sure that each of my faithful and talented shoppers would receive at least one choice assignment before I started scheduling any doubles.