If you don't have Valslides for this one, furniture sliders will work well too. It's the same idea as ring flyes, only with the added element of friction to ensure that your pecs hate you even more.
When you're using the rings, the eccentric part of the rep is the hardest, but if you can pull that off without doing the famed Ring Dip Face Plant (easier said than done, I might add), the concentric isn't that bad because the built-up tension of the rings helps bring you back in.
You don't get that assistance with the Valslides. You have to actively push out on the eccentric and pull back in on the concentric, making both parts of the rep suck equally as bad. Fact is, the concentric is probably even harder than the eccentric.
While it may seem innocuous, it's really an extremely advanced exercise, so don't just jump right into it without proper preparation. Doing so will inevitably lead to either a shoulder injury or the aforementioned face plant, neither of which you want.
Start by doing partial flyes where you only extend your arms out a little bit and progress to full flyes over time. Even with full flyes though, you still want to keep a slight bend in your elbows to protect your shoulders and keep the tension on your chest.
You may also want to start from your knees. Seriously. Laugh all you want, but you probably won't be laughing once you try them.
Be advised that the difficulty of this exercise will vary greatly depending on the surface you're using, so be sure to take that into consideration. It can be anywhere from very difficult on a smoother floor to downright brutal and almost impossible on a rougher surface. A thin carpet works best, making this a great exercise to do at home or while you're traveling.
Quick word of caution: This exercise may not jive well with folks with certain shoulder pathologies. If you find it causes any pain (other than a smoked chest), stop doing it and pick something else. That really goes for any exercise though.