How do you get Moltres in Pokemon Pokemon LeafGreen?

Assuming that you have already gotten the "island" map.

go to 2 island (or which ever island has ember mountain) and make your way up the mountain. Beware that there are a lot of trainers at the base of the mountain that you might want to avoid to save energy for moltres.

Now, make your way up the mountain. Make sure that you have a Pokemon that knows strength, cut, an escape rope (just in case), @#%@% amount of ultra balls and potions / healers. Other than that, ember mountain is really easy to get up because it's literally a one way road. Just follow the trail!

Once you reach the peak, moltres will be there waiting for you. SAVE YOUR GAME in case you faint it or you faint. Now, use a Pokemon that is slightly above lv 50 and slowly lower its hp. It would also be helpful is you can put it to sleep. Do not poison / confuse it cause it would kill itself. For this, I recommend a team made up of snorlax (who knows yawn and have a high hp and defense) a fire Pokemon (so moltres don't kill you with its fire moves) and marowak or scyther (these two knows false swipe at higher levels).

or, you can just use your masterball if you still have it. But I don't recommend this cause you might want it to catch mewtwo later >.<

Good luck!