Apply to the CAA.
Produce a JAR OPS 3 manual including training and maintenance.
Produce a Maintenance Management Exposition (MME) in conjunction with your JAR 145 maintenance organisation for approval by the CAA.
Nominate the accountable manager, Chief Pilot and Operations manager (the last two must be acceptable to the CAA implying they must have experience of AOC operations). Also nominate the Company Training Captain(s) (who must be TREs in order to conduct Base checks/OPCs on pilots). A quality manager should also be appointed.
Up the insurance on the aircraft to cover war risks (the CAA will not allow you to operate without it).
Carry out proving flight (s) with CAA Flight Ops Inspector.
Not sure about the initial costs of setting up an AOC but I believe that it runs into a few thousand pounds.
As well as a commercial licence, pilots must have a current First Aid certificate as well as being current in fire fighting and CRM.
I've probably left huge chunks out but hopefully this will give you a flavour of what's involved over a period of many months.