every situation is different.basically what you do is create the opening for your new box then run your fish tape into the hole where the wire is going to come from down to the box opening,tape the new wire to the fish tape and pull it through.actually it's fun(for me it is anyway).in your case it may be helpful to remove the medicine cabinet depending on how it was framed in.there are a lot of tricks so once you start and run into a problem post your question and we will guide you through.evaluate everything best you can before you start mostly regarding where you are going to tap in(basement or attic)is a switch loop needed ect.