Wanna stack? This tutorial shows how to use the plain header
to connect to an Arduino. If you want to use stacking
headers (https://adafru.it/dsu), don't follow these steps!
Start by sliding the 10 pin, 2 x 8 pin and 6-pin stacking
headers into the outer rows of the shield from the top. Then
flip the board over so its resting on the four headers. Pull on
the legs if necessary to straighten them out.
Tack one pin of each header, to get them set in place before
more soldering. If the headers go crooked you can re-heat the
one pin while re-positioning to straighten them up