Hello
the sentence just a how i play piano is not grammatically correct
( just as how I play piano )
you can say instead if you are asking someone how to play the piano we say ( how do i play the piano )
or if you are showing someone that you can play the piano we say ( look at how i play the piano ) or ( look i can play the piano ) or ( i can play the piano ) or ( i just like the way i play the piano )
