How do you train a dog?

Depends on how old and how well trained the dog and cat are. It`s much easier to do if you have a puppy and a kitten, but can be done at any age. Whoever was there first still gets run of the house - you don`t want him/her to think of the new addition as someone who`s making him/her get locked away. You want to let the new addition out to explore when the resident pet is either out playing or doing something fun (if you have another family member, it`s so much easier to split up tasks). Give them an hour to explore, then they go back to their `room` while the resident pet comes in to check out the newcomer`s scent. If you give me more details of your situation (bringing home a kitten to a dog or puppy to a cat?), I`ll try to help.

Probably when both are about 1 month old. Otherwise it is difficult.