So, let's say I wanted to hatch a perfect Corsola, that had a good nature and was shiny, with perfect IVs.
First, you'd need the right parents; probably a flawless Ditto as one parent, and a male Corsola with the egg moves you want to pass down as the other (or a female Corsola if you don't care about egg moves).
You need to download the programme RNGReporter, which requires the .NET framework if you don't already have it: http://users.smogon.com/mingot/rngreporter.htm
Now, it's a lot easier if you can verify your seed. In Heart Gold, that means either you have at least one Pokémon still roaming around the map (Entei, Raikou or Latias; the more the better), or you've advanced far enough int he game that Professor Elm gives one of three different messages when you call him (he should either mention Pokémon evolution, Kanto, or Pokérus; you need to have gone to Kanto for the second one, and you need to have tried to heal a Pokémon with Pokérus at a Pokécentre for the third)
First, you'd need the right parents; probably a flawless Ditto as one parent, and a male Corsola with the egg moves you want to pass down as the other (or a female Corsola if you don't care about egg moves).
You need to download the programme RNGReporter, which requires the .NET framework if you don't already have it: http://users.smogon.com/mingot/rngreporter.htm
Now, it's a lot easier if you can verify your seed. In Heart Gold, that means either you have at least one Pokémon still roaming around the map (Entei, Raikou or Latias; the more the better), or you've advanced far enough int he game that Professor Elm gives one of three different messages when you call him (he should either mention Pokémon evolution, Kanto, or Pokérus; you need to have gone to Kanto for the second one, and you need to have tried to heal a Pokémon with Pokérus at a Pokécentre for the third)