In order to unlock the bijuu cloak you must first have sealed a tailed beast into yourself. After that, you have to wait for about 5-50 hours. When this time has passed you need to get below 5 hearts. Then the bijuu inside you will say something in chat. You do the same thing again for every other stage. Here is what the tailed beast will say in first and 2nd stage. You do not know when you have to get below 5 hearts its between the 5-50 hours so just do it every 10 hours and it will work. A way to deacctivait the tailed beast mode is shift + the acctivation button. There are 4 stages, 1 you just stand up straight and get a slight damage and speed buff. stage 2 you are next to the floor and you get more speed you will have tails for what tailed beast you have. stage 3 the same as stage 2 but you have a bit more speed and damage and you got bonnes on the skin. Stage 4 you can transform into a huge tailed beast (7 tails gives you the abbility to fly at max stage) and 9 tails give you ying and yang release.
facts: there are only 9 tailed beast, if you got 9 tails inside you get ying and yang release, if you have 7 tails max stage you can fly, you can only have 1 tailed beast inside of you.