Before you begin drafting the instructions, you need to consider who your audience is so that you can take their needs into account. Although you can't "know" your audience in any essential sense—your audience isn't a monolithic or homogenous thing, but rather a dynamic and widely varying group of people—you can make strategic decisions about them that will help you to create documents more useful to them.
The following questions will help you to go about making these strategic decisions:
In asking these questions, you should get a good sense of not just the audience you are writing for, but of the overall rhetorical work that your document needs to accomplish.