All it takes these days is a piece of aluminum foil and a needle. Tape the foil in front of your sensor (without light leaks), poke a small hole into the center

All it takes these days is a piece of aluminum foil and a needle. Tape the foil in front of your sensor (without light leaks), poke a small hole into the center of the resulting spot with the needle. Takes high ISO settings to get something out of that, but it works and is dead simple.