Try looking through the white papers. When I was trying to learn about this kind of stuff I found
this one really helpful to get started. Once you've learned all you can from that try looking at some of the other papers about different gearbox designs like the TechnoKat's for shifting or the Who'sCTEK's for combining motors. If you need any help beyond that try asking around. I'm sure most people here would be happy to help answer any questions you have.