Wiring the LSM303 is easy - like most sensors, it uses I2C - so you just need to wire up:
- 3.3V to 3.3V
- GND to GND
- SDA to SDA
- SCL to SCL
Start with a case. This is a good one: it provides space for a small LiPo battery and an on-off switch, as well as access to the USB connector.
To connect the battery you will need male and female JST cables and a slide switch. Cut the wires on the JST connectors to fit neatly in the case, under the Circuit Playground Express. Connect the black wires together and connect the red wires to two adjacent connections of the switch. Using a bit of heatshrink on each of the connections is almost always a good idea: it helps avoid accidental shorts.
Next, secure the switch to the case with a little hotglue. Once it cools, connect the battery and Circuit Playground Express and tuck everything into place.
Secure the board as appropriate for the case you're using.
All that's left is to snap the cover into place and load up the software.