A change order is a written change to a contract's work, price or schedule, signed by the customer before the changed work starts.
On paving and concrete jobs, the triggers are predictable. The mill drops through soft base. The site has 900 square feet of base repair against a 200 square foot allowance. The property manager asks for the drive-through lane too. A valve box sits four inches below the new grade. The engineer bumps the surface course from 1½ inches to 2, which is a third more surface tons.
The easiest change orders to price are the ones the proposal already priced. With a unit price for base repair per square foot, undercut per cubic yard and extra mix per ton, a change order is just a new quantity at a price the customer has already seen. Work you can't unit-price goes at the time-and-materials rates in the contract.
Changes can go the other way. If the owner drops the striping or the allowance goes unused, write a credit, which is called a deductive change order.
Number them in order (change order 1, change order 2), attach photos of the condition, and put what the change does to the schedule on the form. At closeout, check that every signed change order made it onto an invoice. For the full process, see the change orders guide.