Good advice. And...apply plenty of seed...if conditions are less than perfect, this will insure a good germination and good thick stand of new grass. Its best to use a high-quality perennial rye that is suited to golf or athletic fields. Seeding rate should be around 8 pounds per thousand sqft. Explain the importance--and get the customer to pay for it of course. Tell them no guarantee if anything below 8 lbs per thousand.
I suggest Fastball RGL. Overseed rate is 7 to 10 lbs per thousand, (tees, roughs, and fairways). (Up to 20 pounds for overseeding Bermuda greens in Florida).
Try to use only newly developed modern varieties--something released after 2009. Do not use any seed that does not claim to have good traffic tolerance. Don't accept any seed that does not claim disease resistance. Don't accept any perennial rye that is not in the national top twenty in the NTEP trials. Check the bag label carefully for any sign of weed seeds like "Poa annua", and reject any seed whose label says "Other crop" (that means quackgrass).