Wherever practical, instructions should be written by someone who knows as much as, but no more than, the audience. For any form of instructions to be followed by non-technical users, the writer should assume zero prior knowledge and the best way to ensure s/he does that, is if s/he doesn't have any prior knowledge her/himself. Provided that the writer has a logical mind and the ability to write clearly and simply, s/he can't fail to work out and then write good, usable instructions - because if s/he understands them so will everyone else.

Equally, instructions should not be written by the sales people, the marketing executives, the guys in the lab, the production staff, or anyone else - even you - if there's a risk they might have become familiar with the subject matter. Familiarity can breed if not contempt, at least wrongful assumptions about the audience's existing knowledge. For any product to be used by ordinary folks in the street, try to get the instructions written by someone from a totally unrelated department or even from outside your organization. Failing that, get them tested by one or more typical users who have no prior knowledge of the product, and edit them carefully on the strength of the feedback you get.

There is nothing that will blacken the name of your product and your company faster than a customer like me not being able to put your product together easily.

Although customers like me will get over it after taking a cold shower and asking the brainy next-door neighbour to interpret the instructions, we'll probably remember all those bad things next time we're shopping for the sort of products you sell. And we'll buy your competitor's.