Instructions
Begin your journey through Kanto, catching and training Pokemon. You will need a strong party to defeat at least the first seven gym trainers.
Head back to Pewter City and cut down the tree that blocks your path. Use a Pokemon that has been taught the Cut HM.
Speak with the man on the other side of the tree and he will give you some Old Amber. Its contents contain the fossils of Aerodactyl.
Travel south and swim to Cinnabar Island and go to the resurrection building. The man there will turn your fossil into an Aerodactyl.
Add the Aerodactyl to your party and send it into battle. This will give your Pokemon experience points and will cause it to level up.
Give your Aerodactyl berries, vitamins and other power-ups to make it happy and healthy.