Instructions
Load a copy of "Pokemon LeafGreen" into the GBA slot of you Nintendo DS before playing your copy of "Pokemon Diamond." With both games in your DS, Magby will appear in the wild in "Pokemon Diamond."
Explore either Stark Mountain or Route 227 with your party of Pokemon until you randomly encounter a Magby. Fight and capture as you would with any Pokemon you encounter in the wild. If the Magby drops a Magmarizer item, you can move on. Otherwise, continue fighting Magby until one drops a Magmarizer. This is a rare drop, so this may require multiple attempts.
Use your Magby to fight other Pokemon until it reaches level 30. At this point, it automatically evolves into Magmar. Once it evolves into Magmar, give it the Magmarizer.
Go to a Pokecenter and use the in-game trading system to connect to another Nintendo DS. For these purposes, it is not recommended that you trade with a stranger since you may not get your Pokemon back. Have a friend use a second DS and copy of "Pokemon Diamond" or either of the other two games of the same generation to go into a Pokecenter to perform a trade.
Trade your Magmar to your friend by following the on-screen instructions for performing a trade. When the trade is complete, Magmar will evolve into Magmortar. Trade your friend's Pokemon back for your Magmortar.