I've actually tried to write out directions - and it's so easy to miss something basic, or leave something important out. Only way to make sure they work is to have someone else try to follow them. Of course, it's the same for writing up any type of procedures. I find it often works best to have one person performing the work while another is writing down the steps needed - and then have a third person try to follow them.
If I'm paying for something, I hope the writer/developer took the extra time to troubleshoot the directions - if they are free, then I go with the flow and try to check through and count up and double check it all myself.