Its easy to do and painless for them, rather like cutting our hair.
It does work better if done before they have learnt to fly, but its always worth a try.
Remember only clip one wing per bird. The idea is that you are unbalancing the wing flights.
Then with a pair of nice strong scissors or secateurs just snip the flight feathers so they are about level or a bit longer that the small covering feathers on the wings.
If you dont understand just think which are the longest strongest feathers on the wings and those are the flights and those are the ones you need to be reducing in length. And if you still don;'t understand then post again