In addition to issuing clear specifications for your print job, it is a good idea to provide a PDF file or a mockup of the piece to be printed, even if it is not yet in final form. Having something to review in comparison to the specifications allows us to check the specifications for accuracy and perhaps to develop alternative specifications that will produce the same outcome. For example, we may have a house sheet that is the equivalent to what you have specified but is more economical because we buy it in high volume.
Providing us with a PDF file or mockup is also a good idea when you are developing a budget for a printing project. Although written specifications alone do provide a basis for an accurate quotation, it is very useful to have the file so we can check for technical issues or a level of difficulty that may not be apparent from the specifications alone.