The final design for the piece may be decided as early on in the process as agreement of the contract and schedule, or may be finalised later as a result of a consultation process with a community or a collaborative process with other professionals. You should agree with the artist at the beginning how you wish the final designs to be presented. This could form part of the project schedule.
Remember that this could also be a milestone in the project agreement, which allows for the project to stop here if the final design does not fit the brief or cannot be agreed. It is good practice to make payments in line with the scheduled milestones to ensure monies for fabrication are not paid out before designs are approved.
It may seem obvious but it is probably worth reiterating that site visits may be an important part of the design process and should ideally take place early on. This also offers a good opportunity for artist and commissioner to meet and discuss the project informally during the visit.
Once the final design is complete and agreed then depending on the nature of the project it is likely that some sort of technical specification will be necessary. This could be as simple as the method of attaching a painting to a wall or might include details of all materials and methods of fixings, background information on any suppliers, fabricators, details of site works or foundations needed to install the artwork, and directions for maintenance.
If the artist is proposing to use materials that are not part of their usual practice it may be sensible to secure independent engineering or technical advice to check feasibility or explore options. There may for example, be a need to build in time for testing equipment or materials to ensure they are suitable for the site and purpose.
You should review with the artist at this point, the timetable for fabrication and installation to ensure the project is still on track and that the artist has clear information on such things as other construction or site works that are relevant to the artwork.
For example Technical Specification and Maintenance Schedule see Appendix B ages 19 & 20 in Trafford Guide to Commissioning Public Art