How can I get to Saffron City from Route 6 in Leaf Green?

You have to go to Cerulean City than use cut at the right of the city after you exit the house that was broken into. follow that route and at the end there will be a poke-center and a cave. Go in the cave ( flash would help here but not totally needed) after you exit you come out above lavender city. Go left and go to the top left of that route and the top building. follow that underground the path. When you come out go left into Celadon City. There you go.