To get rotom forms, you first need a rotom. To get this, transfer one from previous games like Pokemon black/white 2, or look on GTS. Once found, locate the Pokemon lab in Lumiose city.
Then, take the elevator to second floor. You will find a bunch of old boxes in the bottom right of the room. If you go up to it and press the "A" button, it will say it's a bunch of old appliances. If you bring rotom, it will say the same things, but it will also say something about rotom wanting to experiance it. Click Yes. It will show you a bunch of appliances - you might be able to make out which appliance is which form. After a form chosen, it will ask you to learn a move. You cannot change your rotom's form if it doesn't learn the move - so learn it. This is how you change your rotoms form into the other 5. To get all 5, you will need 5 rotoms.
Hope I helped! :)