Instructions
Clean the cage from top to bottom, before introducing the new gecko. Clean everything: the cage itself, the plants and cage furniture. Replace the substrate as well.
Arrange the furniture and plants in the tank differently, before you add the new animal. That way, there is no memory of any predetermined territories between the geckos, and it is a new environment for all of them.
Put the new gecko into the cage while the current inhabitants are out of it, so it can adjust to its new surroundings at its own pace.
Add the original inhabitants 20 minutes later. They should now be able to smell the scent of the new gecko all over the cage.
How to Introduce Geckos
Introducing a new pet gecko to one you already have is a tricky proposition, as you don't know how they will mesh together in terms of personality and level of aggression. While you should never keep two males in the same cage -- they will fight each other to the death -- you can house females together and females with a male - provided that you are ready for baby geckos.