To do a shameful slash, you first need a sword that can do a low horizontal or diagonal slash. Once you have that, all you need to is execute the cut near enough to the female that it just barely misses her. If you're too close, you'll hit her.
I recommend testing out your cut a fair distance away if you're not familiar with its reach. Try it a couple times, get a good idea, then start to inch in to your target. It's better to start too far away and miss a couple times than to start too close and hit her. If you need practice, try it out on the weapon-wielding companions since you don't lose any points for accidently hitting them.
If this is for the Ouka job, then make sure you have plenty of practice beforehand. This job is a nightmare even if you've had done a few shameful slashes before.