Step 3: LED cubes (inner frame)
First cut out the bottom and top of the cube row (for me the size had to be 6.5cm by 6cm * 5 cubes + 0.5cm * 6 cube walls), and the six cube walls (6.5 x 6 cm). I used a blade knife to cut alongside a straight piece of wood, which helps getting nice cuts. Glue the cube walls to the bottom. Then glue the top to the cube walls at once.
When the foamboard frame is finished, attach the RGB LEDs to the inside of the frame by pushing them through the foamboard. You can make small incisions to help getting the legs though. At the outside, connect the wires to the right LEDs. Be aware that you attach the wires to the right leg (R, G or B)! It works probably best if you used different colours of wire for the different legs (in a RGBRGB.. pattern) in the last step. Bend the legs towards the foamboard and use some (duct) tape to prevent the legs from touching the circuit later on.
Now cut out two pieces of matte foil that will fit onto the two open sides of the cube row, and glue them to the edges of the foamboard frame. It may be wise to wait with the second one until you tested the software; otherwise it will be hard to replace or adjust the LEDs if necessary.