First, go underground using the Explorer's Kit. You get the Explorer's Kit in the Digger Man's house in Eterna City. His house is right next to the Pokemon center. Use your Explorer's Kit OUTSIDE OF THE HOUSE and you'll go underground.
Here's a vital part of Skull Fossils and Armor Fossils. If you don't have the National Pokedex, and if your trainer ID is an odd number, you can find Skull Fossils underground (though uncommon). If your trainer ID is even, you'll find Armor Fossils. Once you get the National Pokedex though, they reverse. Odd is Armor Fossils, and even is Skull Fossils.
Skull Fossils- Cranidos
Armor Fossils-Shieldon
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Must trade Cranidos appears in Diamond/Platinum (odd ID number) - Revive Skull Fossil
Shieldon.
You cannot get a Skull Fossil (Cranidos) in Pokemon HeartGold.
use your explorer kit then click on the screen then some shiny dots may come in sight go to them and dig in them and it may come skull fossils
You revive it from a Skull Fossil, like in Diamond and Platinum.