All you need to do is find Giovanni and defeat him.
You have to go to Silph Co in Pokemon LeafGreen. You have to go there to get the Silph scope, which you need to advance.
On the 5th floor of silph co.
You receive the master ball in leafgreen after you stop team rocket in silph co. In saffron city as a reward from the president of silph
in saffron city big building
7th floor.
you get it in silph co which is in saffron city. after the boss fight, the leader of silph co gives you one.
Saffron City.
Save Silph Co., Just like in Blue.
its somewhere on floor 9 of silph co
Get the silph scope at silph co in Saffron city. Then you'll be able to fight the ghosts.
You can only get one masterball.the owner of silph co gives it to you.
In Silph Co. in Saffron city. After you get to the top of the building.