it's not that hard, depending on your gearing. a harder gear will make it harder to skid.

truth be told, skidding is really a waste of time (and tires).

if you're really interested, though, as your pedaling basically stand up, move your weight forward over the bars, and lock your legs. everyone has a leg they prefer over the other. as that leg is on the upstroke of your pedal circle basically try to stop the movement. since your weight is shifted forward there'll be less traction on the rear wheel to keep it rolling, which will leave you in a skid.

i would recommend installing a front brake until you're comfortable riding fixed, and even then i'd leave it on.

(i should take my own advice.)

edit: oh, skidding is also a good way to destroy a hub. if you're curious i'll explain how/why it happens.