Choose an appropriate type of illustration, depending on the material that you need to illustrate. If your material is quantitative, a table or graph will probably be the best choice. For schematic illustrations, a line drawing may suffice. To indicate structure, a cutaway or exploded diagram allows the viewer to see interrelationships between parts. Choose the type of illustration that best represents your content.
Decide what needs to be shown in the illustration once you have chosen the type to render. Line drawings, for example, can have fine levels of detail, but only certain parts may be important. The purpose of illustrations is to present specific visual data so make sure that your illustration is clear and clean. Adding too much visual detail, too many labels or unnecessary arrows results in visual clutter that can distract the viewer's eye. Keep illustrations as simple as possible.
Consider the format in which your technical illustration will be ultimately seen by the end user. If the document will be presented online, use web-safe colors, such as red, green and blue, and make sure that the illustration will scale and align correctly on different monitors at different resolutions. If the document will be printed, consider the limitations of your printer; color, resolution and size matter when designing an illustration for print media.
Verify that the illustration integrates well with the rest of the document. If the illustration is to be captioned, ensure that the caption appears as close as possible to the illustration, preferably within one page or screen. If the caption or the illustration's labels use specific terms, use those same terms in the surrounding text; using different terms in the illustration and text will result in confusion. The text should also refer to the illustration specifically; if it does not, the purpose of the illustration can be easily lost on the viewer.