follow the directions on the box I think it's mixing like 1 tbsp of cereal with milk until it's pretty runny for the first feeding for the first 3-5 days to make sure there is no allergy...
Then the teething thing... isn't it wonderful? hehe... my sons been cutting teeth left and right since august... so my suggestion is to give him a bunch of toys to gnaw on just leave them laying around massage the gums because putting counter pressure onto the gums relieves pain alo baby tylenol
If your question is whether you can start cereal when the baby is teething, yes, you can. Babies sometimes teeth for months, so unless they are in pain and don't want to eat, you can start the cereal. I think my daughter actually liked the feeling against her gums, because even when she was crying from the pain, she would still sit long enough to eat some rice cereal over and above her bottle.
What helped us was adding applesauce to the cereal with the formula..or milk, whichever the baby is on. teething is no fun. My grandson like to chew on cold wash clothes. I would a new one in the fridge and give it to him. Also try a teething rag. Im not sure where you buy them. Someone gave us one and it has been a life saver.