AT THE TELESCOPE
Begin by drawing what you see. At first, notice details but don't dwell on them. Try to capture the essence of the object. If you've picked a non-comet solar system object, look at it as you would at a clock face. For reference, note where features begin and end. Try to identify the object's circle.
If, on the other hand, you've chosen a deep-sky object as your subject, begin by comparing it's size to the field of view. Try to capture this ratio in your drawing. If individual stars are visible, sketch them in first and use them as reference points. Then (unless you've chosen a star cluster) begin to lightly shade in the outline of the object.
For either solar system or deep-sky object, compare and contrast features that you see. You can do this by choosing a pair of features and noting:
1whether they are the same height and width
2whether they appear tilted at the same angle
3whether they are the same in terms of lightness and darkness
4how their borders compare
5whether they have the same degree of smoothness
6any difference in color
7where they are with respect to one another
The big difference between solar system and deep-sky objects is that once you are satisfied with a sketch of a deep-sky object, you never have to draw it again. It will not change in your lifetime. (Supernovae in galaxies being an exception.) This is also true of the Moon, but there are enough details on the Moon for a lifetime's worth of sketching. With the Sun, comets and planets, however you have objects which are in a continual state of change.
You never know what use you will have for your sketches.
SKETCHING LOG
If you keep an observing log, it's better that you keep a sketching log as well. This may consist of nothing more than filling in the blanks on your sketching form. Just make certain that you fill them in. When you've filled in a page, file it in some place securely.