How do you get to reverse world on Pokemon Diamond?

You have to get every single Pokemon their is. Then you should go to the spear pillar. Then there will be Pokemon selector. You will have to use your strongest legendary Pokemon. Then you will battle exact clones of your Pokemon. You will have to win. ONce you win there will be a portal right in front of you. Then you have gotten into the reverse world.By going in the reverse world all your Pokemon will be level one million and they will also be shiny.