The process shouldn't imply too many complications, with only two materials it's possible to achieve an almost perfect mirror. The method that we are pointing is based on glass and a fine metal layer like it was done intensively centuries ago.
First, the glass needs to be polished with especial care to avoid any imperfection because any of these could create distortions in the reflection. After this step, the next one is to paste a reflective material like a polished metal of the same size. This works like a sandwich, and finally but not mandatory, it's a protective paint for avoiding corrosion and oxidation of the metal.
As the glass is usually made from silicon in a great percentage, it can be manufactured easily and everyone could experiment if some high temperature oven is at hand. It's really amazing how deserts have incredible quantities of sand (silicon) and solar energy; thinking a bit, mirrors or photovoltaic panels seem there a clear Nature's suggestion.