Start by placing the shield on top of an Arduino (any will do) and threading the four long stacking headers through so they plug into the Arduino headers.

Pull on the shield a bit so that you get a few mm of gold pins showing, then flip it all over and place on a table as shown. It should sit nice and flat

Tack the outer pins of each stacking header. This way they'll be aligned right.

You do not have to solder all the pins! Just do 2 pins per header

Check your work by looking to make sure the pins are soldered

Then continue to the other side

Carefully remove the shield from the Arduino and place flat on the table, long pins sticking up

Now you can solder the remaining header pins!

Check your work, especially the power (5V) and GND pads! Make sure there's no bridged solder connections

If you want to add the on/off switch, place the switch in the slot in the middle of the PCB

You may want to tape it down so it sits in place while soldering

Solder all the pins!

Check your work, there shouldn't be any shorted connections!