Hi AF330,
I thought maybe a couple of engine cutaways might help you to visualize the location of the starter in a large fan engine.
Just at the rear of the accessory drive section is the starter. In the next cutaway, you will see how that connects to the main engine rotor shaft to turn it beginning the process of engine starting.
Now you can see the starter and the shaft that goes from the starter upward to the engine rotor shaft. Note the gear at the end of the starter shaft meshes with a gear that is attached to the main engine shaft. So, as the power coming from the APU in the tail of the aircraft energizes the starter to start turning the main engine shaft, rotational speed of the engine rotors will increase. At a certain rotational speed, fuel will be sprayed into the combustor and ignited. As rotor speeds continue to increase the starter is disengaged and the engine becomes self sufficient with no need for external support. In case you are wondering, the depicted engine is the GE CF6-50 that was used to power the A-300, Boeing 747 and also, the DC-10.
Hope this really helps you.