1. Go to the Specialized Job marker in the Northeast area of Los Santos.
2. Go to the orange Job Selector circle and choose Transformer Repair.
3. Go to the green market icon and purchase Cables, Fuses, and Relays. You need these to complete repair jobs and if you plan on doing this job for a while you should purchase a lot. You can also come back to this location and purchase more when you run out.
4. Wait for a call out to appear in the top right area of your screen, the call out details the supplies you will need, as seen in this image below.
5. Go to the location marked on your map as Job (Transformer Repair) and repair the transformer.
6. You will keep receiving callouts until you switch jobs.