Analyze the angle you have taken. For example, if you are writing about something such as how to repair a guitar, narrow your focus to either the electronics or hardware of the instrument, depending on your area of expertise. The more specialized, the better, since there are probably no shortage of workbooks available that deal with the same subject.
Think about your audience. Age, demographics and even gender are all factors when deciding the appropriate tone to use. The more specific you are, the more accessible the information will be. For example, if you are writing an instruction workbook on how the non-chef layperson can master Italian cuisine, keep in mind that many of your readers want to learn fast and are presumably on the go.
Research and take note of any problems if your workbook is already in use. For instance, you may learn that the material is adequate and informative, but there isn't enough supplemental resource material. If your workbook is not published, have a peer read and review it for any shortcomings.
Embrace formatting. Use of bullets, numbers and various typefaces can aid in providing overall clarity. Granted, you don't have to use bright, colorful illustrations for every page, but keep in mind the visual aspect to keep it interesting.
Use feedback to your advantage, and try to remain open to constructive criticism. You can do this either by consulting those who teach the workbook or by simply providing comment material within the workbook, be it a hotline to call or a website with satisfaction surveys.