Instructions
Go to the second basement level of Mt. Moon and defeat the Super Nerd's Grimer, Voltorb and Koffing.
Choose either the Dome Fossil or the Helix Fossil. The Super Nerd will take the other fossil.
Travel to Cinnabar Island and give the fossil you picked to a scientist inside the laboratory. A Dome Fossil will be revived into a Kabuto, and a Helix Fossil will be transformed into an Omanyte.
Trade for the other regenerated fossil Pokemon from either "Pokemon FireRed" or "Pokemon LeafGreen" to your copy of "Pokemon LeafGreen."
Sweeten the trade for the person you trade with by breeding your Kabuto or Omanyte with a Ditto to make a copy of the creature. Leave a Ditto and the fossil Pokemon with the Day Care Man on Route 5 for a while and return to pick up an egg containing a young copy of the fossil Pokemon. Whoever is trading with you will likely be thankful to own the creature they had to abandon in Mt. Moon.