Instructions
Preparation
Obtain a second Nintendo DS, or find a friend who owns "Pokemon HeartGold" or "SoulSilver" and a Nintendo DS of his own who is willing to trade with you.
Insert "Pokemon Pearl" into your DS, and "Pokemon HeartGold" or "SoulSilver" into the other DS.
Make sure the player of "HeartGold" or "SoulSilver" has a Cyndaquil, Quilava or Typhlosion.
Choose a Pokemon to trade away for the Cyndaquil, Quilava or Typhlosion. You may want to trade away a starter from "Pokemon Pearl," or some other Pokemon that would be otherwise unavailable to the player of "HeartGold" or "SoulSilver." If they don't have any preference, instead feel free to catch a weak wild Pokemon to trade.
Head to the nearest Pokemon Center. Have your friend with the other DS do the same in their own game.
Go to the upstairs portion of the Pokemon Center and talk to the woman at the middle desk to initiate wireless communications between the two Nintendo DS consoles. Save your game when prompted to enter the Union Room.
Select the option to trade Pokemon. Choose the Pokemon you want to trade away from "Pokemon Pearl." The other player will now choose to send Typhlosion. Confirm that you want to perform the trade and wait patiently for the Pokemon to transfer between games.
Exit the Union Room and save your game.
Getting the Egg
Head to Route 218 and use the PokeRadar to find and capture a wild Ditto. Check Ditto's nature and stats if you plan to breed for specific stats on your new Cyndaquil.
Walk or fly to Solaceon Town and visit the Daycare Center.
Leave both Ditto and the Cyndaquil you traded for with the people at the Daycare Center, then leave the building. Talk to the old man outside. As long as he doesn't mention the Pokemon not getting along, there will be a chance of the two producing an egg, though it may take quite some time.
Walk or bike around for a good length of time, occasionally going to check with the old man outside the Daycare Center. After every 256 steps, there is a chance your Pokemon will have bred, though it may take several cycles. Eventually, he will say that he found an egg while watching your Pokemon.
Make sure that you have a free space in your team and accept the egg from the old man. You now have a brand new Cyndaquil egg.