I think there may be some different answers on this since nobody plays exactly like anyone else, but here goes.
Keep the wrist fairly loose and work first on strumming something easy like the 5th fret D-G strings. You'll see that you have to adapt to avoiding the E and A (assuming 4 string for this) as well as muting them with the left hand somehow. I've seen guys bring their thumb around for this, but I prefer in this case to do something like hold both notes with the 3rd finger while resting the 1st and 2nd on the other strings. I hit the strings with my nails on the downstrokes and outside edges of my fingers (usually first 2 or 3) on the upstrokes.
To get a flamenco effect, make a loose fist and let the fingers release from the thumb...but this only works on downstrokes and does not require much movement of the arm. Once you get the feel, doing one string (any and switching) won't be a big deal.
Work with a metronome btw and don't forget things like triplets.

Hope this is of some help, or at least understandable!
Mark