Best time to clip is right after a molt when all feathers are fully grown out. Clipping does not hurt when done properly, it's like cutting your nails. Look for blood feathers among the wing plumage as you do not want to cut them. Clipping will need to be done each year as the cut feathers are replaced when they molt.
For a perminate grounding...pinoning is what you'd may rather do, here's a good how to PINION page.
A variation to pinioning by Neil see Flight Operation
Two ways to clip the wing:
The wing streched out
Clip only one wing...
Also called the "beauty clip, Clip only one wing...