Straight running downhill is mainly a question of confidence and balance. As said before, it is better at first to avoid straight running down a steep slope, because the Skis may go so fast that the beginner is quite incapable of keeping up with them and a fall at very high speed is somewhat upsetting and may temporarily shake your nerve.
Choose a low gradient of about 12° or 15° where you can see the out-run which should be on to level ground or even a gentle rise so that the Skis gradually pull up of their own accord. Soft snow is the easiest and confidence may soon be won in this.
Stand upright or bend the knees, but do not bend at the waist. You should feel as though on springs and you want your weight should be well forward over your feet so that you can keep up with the Skis. Standing in tube or bus, facing the way you are going and not holding on to anything is very good practice at home. You will notice that a bus conductor usually gives with the movement of the bus, so that he is prepared for whatever it does. So with Ski-ing. Look ahead and see what the ground is like, and then suit your balance to what is likely to occur as the ground rises or falls. This soon becomes automatic but it needs thinking out at first.
When the snow is hard, practise side slipping, because it will help you out of many difficulties and once you know the feeling of it, you will find that it replaces the downhill side-stepping, which is so slow.
On hard snow, it is possible to go down broadside on by merely standing on one's Skis and turning one's outer or lower ankle outwards and one's inner or upper ankle towards the other, so that the Skis are lying flat on the snow, instead of the edges biting into it. Push off with your stick from the slope above you and weight your heels or your toes according to whether the Skis are sinking in front or behind. Have confidence, keep upright, lean away from the slopes and let your Skis slide and don't blame me if you suddenly slide into a soft patch of snow, which stops the Skis dead and you fall head downwards. This is all in the day's work. If the surface of the snow is uniformly hard you will slip down without difficulty.
Seriously, side slipping is a huge help and should be learned at once. Mr. Caulfield gives first-class instructions, which are easy to follow in detail.