Here's one of the better You Tube Tutorials about clipping wings.  It's not horribly hard and not painful at all as long as you only clip the large flight feathers. 

I too prefer to only clip one wing, but some do prefer to do both wings.  They can still fly a bit but not very well or very high.  It really just depends on your circumstances.

Use caution if you use one of the methods demo’d where you flip the chicken upside down.  That can be a bit risky because they can get material in their crop into their lungs.  Most of the one’s I saw were pretty careful to allow the bird to arch its neck which they will instinctively do if you let them and keep them calm.  They do get very calm, but there’s risk to it.  That’s why I like this lady’s demo best.

Well, if they have flight feathers, and they can fly, I imagine you can clip their wings!  Just keep an eye because when they molt and get new flight feathers, they'll start to fly again. 

I meant to say also that I don't see them getting up in a tree as a big deal, I had some young birds get into some low branches of a dead tree, but they didn't find it very interesting for very long so they came right back down…  of course, getting into a yard with a dog that might hurt (eat) them is of course a bad thing.