T&M suits work that no one can measure in advance. Repairs, hidden conditions and some change orders are examples.
A plumber opens a wall and finds corroded pipe. The owner agrees to pay hourly rates plus material with a set markup.
Agree on the rates, the markup and who signs the daily tickets before work starts. Get each ticket signed the same day.
Some contracts set a not-to-exceed amount. Track hours against it and warn the GC before you reach it. Read the contract for the allowed markup.