My preferred approach is to switch a condom to my pants pocket or grab one out of the nightstand and tear it open, right when we get somewhere private, while she's distracted (before I've started escalating). Once it's time to wear it, you just slip it out of nightstand or pocket, take the condom out of the already-torn package, give it a quick look to see which side is the "on" side, then slip it on and roll it up your member with thumb and forefinger. All the while you continue to finger her with your other hand.
In a pinch (if I didn't have one open already, or if the condom isn't coming out of the wrapper) I will bring the condom wrapper up to near where I'm fingering her, switch to my pinkie finger on her clitoris, and use both hands. Then finish putting the condom on with one hand while the other hand focuses on stimulating her.
I've heard a few guys talk about tearing the condom wrapper with their teeth. There's probably some technique to that, but the few times I've tried it it hasn't worked as hoped.
But opening one up as soon as you're somewhere private, and making sure it'll be within easy reach (preferably in your pocket, in case you move around and don't end up escalating next to where you'd usually put a condom), is the easiest way to do this.