The same way as fire red!
first get them on firered/leafgreen from the seven islands, then migrate them
you need to fix that professors machine on the sevii islands (the seven islands) the ones that you unlock after the league
Use vs seeker and train at six/seven islands, maybe......
There is no island 9 in either Pokemon FireRed or LeafGreen. Whoever told you this is very off. There are Seven Islands (Sevii Islands)
Johto is inaccessible on Leaf Green although Islands Four to Seven contain Johto Pokemon.
On Seafoam Islands.
You can't go to Hoenn on leafgreen, but after you beat the elite four you can trade with hoenn and go to the sevii islands. (all seven of them)
Seafoam Islands
On Cinnabar Islands.
There are 7 non-event islands.
actually there are seven
I don't really know about Pokemon leafgreen, but in Pokemon red you can get it in seafoam islands. Hope this helps!