Skiing downhill on cross-country skis in theory should be no different than on alpine equipment. In the fast-evolving "nor-pine" fad, in fact, excellent skiers are proving the correlation by conquering steep slopes on light Nordic equipment.
For average skiers, however, not having stiff, high boots makes a big difference, as does the absence of metal edges on skis which also may have no side-cut for easy turning. The sense of having little control over those long skinny things can be overwhelming as you start down a hill.
What to do (besides sitting down rather quickly, which is the stopping technique used by many cross-country novices and some not- so novices!)? The first rule of thumb has to be "stay low ans stay loose."
If your knees are bent and your hands are in front of you, you'll be in a good position to absorb shocks and apply turning or stopping pressure on the skis. If you go downhill stifflegged, the first bump is likely to put you off balance on your heels, or worse.
In the proper downhill position you can put your skis into a wedge (or "snowplow") and by applying pressure on one or both skis, you can either turn or slow down. Some skiers at times even use their poles as canoe paddles, grasping them together with both hands and "breaking" against the snow, dragging them behind on one side or the other.
If you're in a track and coming into a turn too fast, you can lift the outside ski, pointing or stemming it in the direction of the turn, while applying pressure on that ski. Keep your hands in front and t hink of them as steering you around the turn.