Help

First you need the Explore Kit from Eterna City, go underground and dig in the walls for Plates. There are about 16 different plates that Arceus uses. Sky Plate is the Sky form an so on.

You give it a plate. For example, you give it sky plate and it changes color and is now the flying type. Also, it can learn fly! But, it can only use fly when it has the sky plate.

You capture the Arceus first,then collect plates they look square and are different colors then attach them to your Arceus. He will then change color and type. I think there are a total of ten plates or more,just to get it right theres a plate for every type except normal.

You see the different elemental plates you sometimes get? For example.. Flame Plate, Zap Plate, Draco Plate, etc. Well, give any one of these to Arceus and it will change it's type. For example if you give Arceus the Flame Plate it will become a Fire Type, or if you give it the Draco Plate it will become a Dragon Type, and so on.

Hope this helps,
~Geezerguy

Plates, like the flame plate, the dread plate, and so forth. It doesn't really change form, it changes color, but the biggest change is the type. For example: A dread plate will make Arceus a Ghost type

Hmmm if you mean getting him to different color and type just give him a plate

Like everyone else says, just add a plate
P.S: get arceus in your party then go to oreburgh mine, canalave library and the celestic shrine. There will be a hiker guy @ oreburgh mine and canalave library talk to him and he will tell you about plates (he will give you the flame plate @ oreburgh and he will show you a book you can read about all the different inscriptions on the plates). Then @ celestic town cynthia will talk about what the cave painting mean.