A few small habits make coloring smoother and the results neater — whether you are five or fifty.
For the cleanest lines, print at 100% scale (avoid "fit to page" if it shrinks the image) on plain white paper. Standard letter or A4 paper both work. Heavier paper holds up better if you plan to use markers.
Start with light, even pressure and build color up in layers. It is easy to make a color darker, but very hard to lighten it once it is down. Coloring from the edges of a shape toward the middle helps you stay inside the lines.
With colored pencils, lay down one color, then go over it lightly with a second to create a smooth transition. For a soft glow, finish an area with a white pencil to gently blend the tones together.
There is no race. Put on some quiet music, take your time, and treat each page as a small, finished project. The point is the doing, not just the result.