These sites can be created almost anywhere, but choosing a good location is important since only Siege golems can be moved, and they are easily destroyed.
When build sites are created any player on the constructor's team can add supply to it, increasing its health. When its health reaches 100% it becomes a fully functional siege weapon. 1 supply takes approx. 2 seconds to be applied to the build site. With enough players building all at once the process will be quicker.