Replacement assembly directions are usually downloadable as PDF files from the product manufacturer's website. If they are not on the website, contact the company's customer service department by phone or email to request replacement instructions.
Some company websites, such as Ikea.com and West Elm.com, let customers select from a list of their products to access assembly instruction documents. Lifetime.com requires the customer to input the model number and product ID in order to obtain the assembly directions. Sauder.com requires a scan of the product's bar code or the model number as part of its request form for a replacement instruction book.