Be clear about the job you need to have done. Provide as much information as possible and add photo's to make your job more clear to understand. Try to describe the materials or brand as clearly as possible and avoid vague terms. Document as much as possible, thereby making it clear what both parties have agreed upon.
Also think about work preparation, removal of leftover material. Be aware that the agreement is for the benefit of both parties. When an agreement is reached you cannot halfway the work change your mind without there being financial consequences
Also it is a good idea to document supply dates, that way there is clarity on what needs to be supplied and when.