You can do it in the QoS page. Match the interesting packets with the classifier, assign bandwidth with the class manager and finally assign the class on the interface manager.
I suggest you take a look in the documents first before asking http://www.zeroshell.net/eng/documentation/
I have done what your dimensions, but the QoS classes and calsificador is for the entire network (high, medium and low), I need to limit bandwidth per IP – something like traffic shaping for each user or ip of the network.