Work through one ruler workbook yourself so that you have an answer key to work from. You should understand the progression of exercises and proofread for any errors in the text.
Introduce the ruler to your homeschooler, talking about what inches and centimeters are and how they are used. Ask your homeschooler to practice taking measurements of her body parts or basic household items such as silverware, paper or books.
Show the homeschooler the ruler workbook and give an overview of the lessons that will be covered. It will be easier for him to engage each lesson if he has an idea of what he is working towards.
Set aside time each school day for your homeschooler to work on one lesson in the ruler workbook. Sit next to her, helping her as necessary. Check her work against the answers in your own workbook. If she is having trouble, give her extra worksheets to work on that pertain to the day's lesson.
Write a test covering the concepts in the ruler workbook and allow your homeschooler to study his workbook for a day or two prior to taking the test. Give the exam in a closed-book fashion, correct it and then staple it to the back of the ruler workbook. The workbook and exam can be kept for later review and as a record of the measurement unit that you covered.