Yeah I did that... they're real good grab bars which I mainly use to tie stuff off to when hauling some thing like a toothbrush or jacket rolled up... ain't much room.
The cut pattern is already on the back of the rear cowl, use a dremel and be careful to cut to the correct line. This assumes you got the Yamaha grab bars.
I have hooked many shirts on the rear points of the bars and ripped them. Even caught my shoelaces on those points one time when bump starting it down an incline, and had to drop the bike to get loose (no, it didn't start, I left the key on and drained the battery ) I tuck my shoelaces into my boots now.