DISCLAIMER: Stingrays are venomous animals. This is only to be used as a guideline. Interaction with a venomous fish is your choice. MFK is not responsible if you get hurt while trying to trim your stingray's barb.
I found a few more old pics showing the trimming of a young Southern stingray, so you can see how I hold the tail for a mid-barb type ray. Most benthic rays are equipped this way (Yellow, Round, Cortez, Atlantic etc...) This would be similar to FW rays as well. You can see I've got a neoprene dive glove wrapped around the tail. These rays (and many others like them) have small spines (not venomous, just sharp) all the way down the length of the tail. The glove is to protect my holding hand from being poked. I've learned over time to not actually wear the gloves. If a barb does go through, then you either have a glove nailed to your hand, or you have to struggle to get a bloody glove off with a barb stuck in it and you.