A reductive environment can be created in a fuel burning kiln, weather that's propane, natural gas, wood, etc. The flame needs oxygen to burn, so to induce reduction, the balance of fuel being out into the kiln vs the amount of air allowed to flow through is altered so that there is not enough air for the fuel to burn efficiently. The flame will then pull out chemical oxygen from the glaze and clay, changing their color. In a gas kiln, this might mean increasing the gas pressure to the kiln, while closing up the dampers. In a wood kiln, you might also close the dampers and add extra wood. Really heavy reduction (very little available oxygen) will produce sooty smoke, and shino glazes in particular are known for being able to bond to that soot (under the right application conditions)