Outline the material. Use the first couple of pages to let the user know what the instruction manual will cover. Place the table of contents in this section so users can quickly and easily reference information.
Provide a list of supplies or resources needed to complete the project outlined in the instruction manual. Use pictures, if possible, so users can easily identify the necessary items.
Provide step-by-step instructions. Be concise, but thorough. Use simple language and sentence structures. Along with each step, provide tips or reasons that step is necessary and why it is listed in that particular order.
Place legal disclaimers and any other material that is not relevant to completing the project in the back of the instruction manual.
Test drive the manual. Hand the manual off to someone who has never seen the manual before. Have him use the manual to complete the project, without outside assistance. Ask your tester to write down any concerns, questions or comments about the manual and the process or assembly. Look at his project to see if he completed it successfully. Repeat that process with multiple testers, preferably unknown to each other to keep the results untainted. If multiple test users have trouble with the manual, pinpoint where the trouble is using the testers' comments and rewrite the manual. Repeat the testing process until users can successfully use the manual to complete the project.