Instructions
Acquire a Ditto. In "Pokemon Diamond," you can use the Pokeradar to find a Ditto on Route 218. Track the Ditto using the Pokeradar and engage it in battle. Whittle its HP down and throw a Pokeball as you would with any other Pokemon you were trying to capture.
Bring the Ditto to the Pokemon Daycare in Solaceon Town. The Pokemon Daycare is the first building on the left of the main path upon entering the city. Use the PC and drop your Ditto off at the Daycare. The process is similar to dropping Pokemon off at any Pokemon Center.
Trade one of your Pokemon for a Cyndaquil. Trading can be done at Pokemon Centers. You need a second DS and a copy of another Pokemon game for the DS.
Follow the Pokemon trainer for each game go into the Pokemon Center and speak to the woman at the trading desk. Follow the on-screen prompts to initiate a trade for a female Cyndaquil. If you already have a female Cyndaquil from a previous trade, skip this step.
Return to the Pokemon Daycare at Solaceon Town and drop off the Cyndaquil. If you have any other Pokemon in the daycare, take them out. Leave the two Pokemon at the daycare for a few days and they will eventually mate and create a Cyndaquil Egg.