Wanna stack? This tutorial shows how to use the plain header to connect to an Arduino. If you want to use stacking headers, don't follow these steps!
This step is for all Arduino and compatibles. Please note that we use the hardware SPI port (the 2x3 pins) for talking to the Bluefruit LE module. If that port is not available you'll have to jumper the SCK/MOSI/MISO pins to other pins and use 'software SPI'!