Mastered Ultra Instinct is just a supermove, it's not a mode nor a permanent transformation.
You need to lose a match, go Ultra Instinct, then wait for the blue bar to fill up, win a round and press Start. It's a counter attack.
How to transform:
Down, Down, Z.
Spoiler: commands (click to see content)You'll have to go through all the forms, from normal to SSJBlue Kaioken. When in SSJBlue Kaioken, wait until his blue bar gets empty. You should see Goku tired, that's when you do D,DF,F, z.
It's a counter that makes him go Ultra Instinct.
When in Ultra Instinct you have to win a round, then make sure the blue bar fills up and press Start, it'll be a counter that makes him do the Mastered Ultra Instinct supermove.
For everyone who doesn't have the character, he's talking about this: http://mugenguild.com/forum/topics/son-goku-dragon-ball-super-280gou-20180522-183761.0.html
Edit: since the character got updated I don't know if what I said holds true.