Creating accurate assembly instructions is surprisingly difficult.

To be able to create them, you have to know how to assemble the product, BUT that also means you've been blessed by the curse of knowledge — now you have to perfectly serialize your knowledge into instructions WITHOUT omitting any implicit "obvious" steps that you may have taken due to already being familiar with the product.

Very few assembly manuals get this right. In the case of furniture, the orientation of items is often omitted, after all isn't it obvious that this side goes up? This is how I ended up with some exposed non-laminated wooden board parts in my bed. Sigh :)

QA testing is the key here — write the first draft, have someone try to assemble the thing as naively and true to instructions as possible, fix mistakes, repeat.