Step 3: Start Placing LEDs!
Fill the first row of holes with LEDs! I like to keep the LEDs in a bowl to make them easier to grab.
Make sure the LEDs are oriented to match the outline on the matrix board. The orientation flips on alternating rows. The flat on the side of the LED can be hard to see, so I just remember that the LONGER lead is opposite of the flat side.
You might need to wiggle and twist the LEDs a bit to get them all the way though. The holes are a tiny bit undersized, but this helps hold the LEDs in place when you turn the board over to solder on the back.
The matrix board is NOT symmetric, so make sure you start at the bottom row as marked by "1".