I've found more work that needs doing. How do I add it?

Stop, before doing the extra work. Send the customer a Trady message describing what additional work is needed, your additional cost, and any timeline implications. Wait for them to confirm in writing through Trady messaging before starting the extra work. If you do the extra work without their written agreement and they dispute it later, you're unlikely to be paid for it — because there's no record of authorisation.