I just bought a new house and the basement is not completed,I want to paint and put the knock down ceiling,can you tell me the steps to do the ceiling?

Explain knock down,in your own words.
A suspended ceiling?
A framed drywall type?

If it's a true knock down ceiling, you'll need a hopper and compressor to shoot the texture up as you would if you were texturing a ceiling, you can get these at a paint store or Lowe's , home depot, for around $80 for the hopper, as for the compress it needs to run it, you'll need to find one that has a large tank to keep a constent air flow to the gun while texturing,
In my experience a knock down is more sheetrock much than anything else, you may be able to buy a premixed bag of it. If not you'll need to water it down just enough to pass through the gun and on to the ceiling , if it is to thick it either will not shoot at all or just spurt. Another thing is it is really messy. I always double plastic the floor and tape it down to. I use a single layer on the walls, make sure you use a release tape so to not damage the paint on the walls. As for the tape on the walls it needs to be taped all the way around not just enough to hold it up. When you cut the air on to the gun it will blow the plasitc right off the wall if not done correctly, good luck

To keep from having to paint the ceiling later, mix the laytex paint in with the mixture

I agree with Glavine Messy,Messy,Messy in addition to covering walls and floors cover yourself head included I wear ski mask under ball cap when I spray texture and coveralls
Yes its hot sometimes but better than paint in the hair and all over your clothes especially if you still have to drive home.