Once you get the Secret Key, go to the Eterna Galactic Hideout. If you have emptied Eterna of Team Galactic, you can go to one of the rooms and use the Secret Key between a bookcase and a wall. This will open up a whole new room in the building

When you go into the room, you will see 5 different appliances on the floor. If you have Rotom with you, you can go up to any of them and select it. Once selected, Rotom will possess the appliance and thus you now have one of the new Rotom Forms. However there are a few things to note.

When you select the appliance and Rotom takes possession of it, Rotom will have the ability to learn a new attack. This is dependant on the Form that is is taking. This attack can only be kept upon this particular form. Once you change to another form, Rotom will forget that attack and learn a new one.

To change back to Rotom's normal form, all you have to do is select the area that the appliance once was and Rotom will revert. Although Rotom's stats will lower, this is the only way to get the original Rotom back. Rotom will also forget any specialised attack it had

Below are the forms of Rotom, the items you need to select to get to it and the attacks that each form knows.