all the way at the top, in the middle of the new growth... spread the growth apart, and clip as much of the center plant mass as you can. I'll see if I can't find a picture...
that's what it should look like a week or so after you do it, because the top leaves spread apart... do you see now what you have to do? I can try and draw a picture for you if you really need it...
yup... you wanna top it at least a week before flowering... if not sooner... at the point you're at you'll just stunt bud growth for a week and kill the yield