You have them take care of two Pokemon that can be bred together. The egg that results depends on the Pokemon there and sometimes the items they hold.
you can put them in a day-care it can have an egg.
You go inside the Daycare house next to the Day care man and talk to the Day Care lady. You have to pay money to get your Pokemon back. If your Pokemon has had an egg, talk to the Day Care man and get the egg. Then, you can get your Pokemon back.
If you put a male Pokemon and a female Pokemon in the Day Care in Solaceon Town, they will eventually have an egg. That is breeding in Pokemon Diamond+Pearl.
get a ditto and what Pokemon you want to be in the egg to the day care put them in the day care together then later they will say they found a egg or instead of a ditto bring two of the same Pokemon opposite genders and do the same
leave two banettes at the day care center and there will be a shuppet egg.
If you left two Pokemon that are of different genders and are of the same egg group, you can get an egg from the Day Care Man outside of the Day Care after you've left the Pokemon there for a while.
You could get a ditto and put the ditto with whatever Pokemon you want to clone into the day care center. After a while, when you go to the day care and talk to the old man he will say that he found your Pokemon holding an egg. The egg will hatch and it will be the Pokemon you wanted to clone.
There are two ways to figure it out: Occasionally pass by the day-care. If the day-care man is in a different position than usually, then your Pokemon laid an egg. You can also get an app for your Poketch from a man in the day care. This app will tell whether your Pokemon has laid an egg, and what level both Pokemon are.
if you put two of the same Pokemon but one male and one female. if you wait a little bit then the Pokemon will you know they will have an egg and you go to the man outside of the day-care and he will tell you that the lady inside has an egg for you but you can only obtain the egg if you have space in your team. then if you walk around with it then it will hatch into the pre-evolved Pokemon of the two Pokemon that you put in the day-care. hope this helps. ;)
this is how to double Pokemon in Pokemon diamond you need a ditto and the Pokemon you want to double (if the Pokemon is evolved then the evolution cycle starts over) you put the Pokemon you want to clone in the day care in soloceon town you wait until you get an egg after you get egg hatch it then you get a copy of that Pokemon
well theres not really a code but if you put in two Pokemon the same type but female and male they will have an egg eventually.
you put a female Pokemon and a male Pokemon together in the day care and run around and then talk to the man outside the day care if he doesn't give you an egg run around a lot more.