Ignition key tumbler replacement instructions please?

We had a mechanic take a look at it and he suggested it was a starter. So we bought a new one he installed it work for a little bit, but the next day the same thing happened again. He looked at it again put a new ignition switch work for a little bit but the next day same thing the only way it turns is if we give it a jump but next time I try to turn it back on it won't let me. Mechanic suggested we have a locksmith take a look at it but I do not trust him anymore and we have spent too much money on him for nothing. A week before all this happened my stereo was stolen and was yanked pretty good so don't know if this has anything to do with it.