you don't
you have to catch a dragon Pokemon holding a dragon scale
Attach a dragon scale and trade
On Route 13 by someone or Route 18.
catch a dragon Pokemon such as horsea and Bagon until you get one holding a dragon scale.
The dragon scale is located in Mt. Mortar
To get a dragon scale is to catch a horsea or a bagon.
You can evolve Seadra by trading it with the Dragon Scale
The dragon scale evoles seadra to kingra.When you trade seadra holding a dragon scale it will evolve to a kindra!
In Pokemon, A dragon scale is used to evolve Seadra into Kingra through a trade.
Seadra evolves into a Kingdra when traded holding the dragon scale.
Seadra can never level up to evolve. You have to trade it when it is holding a Dragon Scale item. - Pokemon Guru
seadra