Purchase rolled sod that you can roll onto the area to be used as a putting green. Grass seed often has a lot of weed seed mixed in with it. If you do choose to use grass seed, keep it moist and covered until it sprouts, then spread more seed until it sprouts again. Once the area is covered in grass, maintain it by mowing it short and use weed killer. Fill in any open areas with more seed and repeat. It should be ready for putting in three weeks at the most.