1- Steal all the Bijuus, or obtain them by sealing them
2- Once you have all Bijuus, go to the cave where the Gedo Statue is
3- Once there, all you have to do is fuse all the Bijuus
4- When you get the Juubi, you can choose between Obito's Juubi and Madara's Juubi
NOTE: Current updates have secret reqs that you have to fulfill before fusing the Bijuus.