Add sugar water to powdered sugar and stir until completely combined. Mixture will be thick. The consistency you're looking for is a slowly dripping icing that disappears into the bowl within about 12 seconds. Add water by drop if you think it's too thick. If it becomes too thin, add more powdered sugar by the teaspoon. Use immediately.
If you need to wait to use your icing: cover tightly, and use within 1 day. You will need to stir it up again and possibly add more water before using. Re-test the consistency before using.
If you want to color your icing, I prefer gel colors: my favorite brand is AmeriColor. They have a huge and gorgeous collection of colors in a 12 pack or a 50 pack.