▲ The box balls in front have just been clipped, but the large one at the rear waits its turn.
For good results you will need really sharp shears, one-handed shears or garden scissors for detail and some kind of mat or cloth to catch clippings.
Timing
The best time for clipping box is June as the plants have finished their growth spurt. If you do it earlier they will continue growing and end up looking shaggy in just a few weeks, so needing another clip. If you can’t tolerate waiting until June you can clip as early as you like and as many times as you like throughout the year.
Technique

The blades of the shears need to be kept parallel (flat) to the surface being pruned. For small box balls – up to 3ft, this often means standing over them, partially straddling them. Keeping the blades flat against the surface of the topiary I work in arcs from the centre at the top, making sure the shears are vertical when I reach the sides of the ball.
I stop, stand back and assess the overall shape frequently. Once I have finished working all the way round I use one handed shears or garden scissors to tidy up the final details.
Catching the clippings

With the ever present risk of box blight threatening gardens in the UK, it is important to clear away as many of the clippings as possible.
Clippings can be collected using dust sheets, polythene or as I am doing here door mats. What ever you use, it needs to be able to fit between surrounding plants and tuck under the edges of the box.

I prefer something semi-rigid yet flexible — like these door mats — as they are easy to fold and tip out the cuttings.
Final Result

These box should stay neat and tidy until the end of the year.