Step 1: Solder the LED matrix
The cathodes are connected together to COL A (column A, not the drink). The anodes are connected to ROW A and ROW B. You can use ULN drivers, or generic BC547 and BC557 transistors. We use transistors, the Arduino drives the base through a resistor of 1K. The transistors of the two rows drive the leds through a transistor of 220ohm, for a current of 20mA. You can use higher currents because the brightness drops during the multiplexing cycle. If you turn on the led for too long, you risk burning it.
The photos below indicate the way you solder the leds. Be patient :-)
On the front side, solder the cathodes together.
On the back side, use tape to isolate the cathode columns and unused islands. Then solder the anodes together.