Copy the config from the existing stack to the new switch
You do not copy the config manually between stack members. Just put the same version of IOS on the new member as you use on the rest of the stack, and connect it to the stack. The config will automatically be pushed to the new member.
Set stack member number and priority on new switch
Physically connect the new switch to the stack using the stacking cable
These are reversed. You can't pre-configure the switch member number and priority. You will change the priority of the stack member after it's connected to the stack, through whichever member is the stack master.