To use the shield, simply plug it on top or bottom of the D1 Mini, making sure that the notched corners of both boards align.
It is physically possible to plug the shield up-side-down, but it will not work then. Fortunately, since the pins that provide power will then not be connected to the shield, it shouldn’t result in any kind of damage.
Next, connect the servo plugs vertically into the servo sockets, paying attention to the order of wires: the brown or black wire needs to go to the bottom pin, and the white or yellow wire to the top.
You can stack this shield with any other shields that either don’t use the pins D1 and D2, or use them for I²C and have a different address than set on this shield. If you want to stack more than one servo shield on the same board, you have to change their addresses with the jumpers on the back.
This means that you can’t use it with the RGB and Relay shields, as they use those pins for other purposes than I²C communication.