We'll start with an assembled SmartMatrix Shield. For the pictures in this guide we used a SmartMatrix Shield with female connector to plug the Shield directly into the panel, the terminal block power option, and we added the diode and cut the Teensy VUSB trace to power the Teensy from the external power supply. You can use the other assembly options listed in the SmartMatrix Shield assembly instructions if you prefer.
To make for a lower profile install, you might want to swap out the straight male pins for the expansion header with right angle pins if you have them, like we did in this guide.
Refer to the linked instructions to assemble your shield:
Note that the below instructions were written for SmartMatrix Shield V1, and that is the board used in the photos. The pins names below will similar on all shields, but the pin positions on the board may be different depending on your shield.
Because the Teensy 3.1 runs at 3.3V, it can be connected directly to a microSD card without any level translation or voltage converters required. You probably have some spare microSD to SD adapters that came with microSD cards. You could also use a microSD or SD breakout board if you prefer, just match the pinout of the board you purchase to the diagram.
You'll need a microSD to SD adapter card, 7-pin straight header pins, and a 3" six-conductor female-female jumper cable. You can use a longer cable if you need to for your build, but you may have to slow down the SPI signals if you have trouble reading the card through long wires.
Solder header to card
Melt a bit of solder to the first gold contact, skipping the recessed contact which isn't needed for this build.
Add the 7-pin header, and melt the solder to make a good bond between the gold contact and the pin.
Continue soldering at the other end of the card and repeat for the remaining pins. There are two pads close together at this end. Make sure you don't short the two together while soldering.
When you're done it should look like this. Inspect to make sure the solder is connecting each pin and contact.
Connect microSD adapter to SmartMatrix Shield
Connect the six-conductor cable to the pins, skipping one pin as shown in the photo.
To avoid tangles it's best to connect the microSD adapter to the SmartMatrix Shield expansion header with the adapter contacts facing up and starting from the 3V3/GND signals. Then work your way left connecting the remaining wires along the expansion header.
When using right-angle pins for the expansion header, the labels get covered up. Use this diagram to identify the pins.
When you're done wiring, you can give the cable a slight twist and mount the microSD adapter to the plastic in the center of the display with some double stick tape.