Whisk until everything comes together. You need the texture to be just right. Try making a line with the icing over a dish or piece of paper. It should stay in that shape rather than spread out and fade. At the same time, it needs to be easy to pipe. If it's too hard, add a tiny (TINY!) bit of water at a time and whisk it in. If it's too liquidy, add more sugar, always through a sieve or colander.