I know there's like 10 versions already but none were entirely correct so I added my own one. Like in the last line there's 3 wahs but noone ever cares to add the third wah.

To play -6 and -8 in quick succession try tounge blocking the holes you don't need. Always block the 7.

-6-8-6-8-6    -5 6 -4

-6-8-6-8-6    -5 6 7  

-6-8-6-8      -5 5 -4 4 

-6-8-6-8-6    6 -4-4