What is a good way to level up on leaf green?

If you already have a high leveled Pokemon, you can give Exp. Share to the Pokemon you want to level up and also keep it on the first slot but switch it out quickly before it gets hit. When it sits on the first slot the experience it gets with Exp. Share is always higher. However, I sometimes just make them hold Exp. Share and put them in another slot, due to some foes knowing quick-attack, extremespeed, etc. Those attacks will KO your low leveled Pokemon in a flash.

If you do not have Exp. Share because it is too early in the game, you need to set the Pokemon you want to train on the first slot and switch him out as soon as battle begins. Like I said, this Pokemon will still be vulnerable to quick-attack and such, but there is just no other way to level quickly.

Be wise, if you are trying to level up a magikarp, don't go to the power plant to get it jolted! Go to a place in which your magikarp will have a type advantage at least. Make sure the other Pokemon in your party are not weak against the type you'll be fighting. Be patient, the Pokemon games are designed to be extremely difficult.

If you have access to One Island (after you defeat the Cinnabar Island gym) I suggest you go and train on Kindle Road with your rookie(s). Hopefully you can get enough experience there to beat the Indigo Plateau, which is a heck of a very tough battle!

Your weakest Pokemon should be at least level 48, and the toughest should be at least level 55 to succeed, and bring lots of revives!