Trim the pins, so that the top of the board is completely flat.

If you are going to use the low-profile female headers on your Feather boards, you can put the plastic back on the pins, since they only need to stick out very little. If you are using regular headers, leave the plastic off, this way the pins can go deeper into the female headers and the whole thing will be thinner.

Insert the bi-color LED matrix into the holes. Make sure to make the text on the matrix face down, as pictured here. You will need to use your tweezers to make every pin fit into its hole. It takes some patience.

Push the matrix against a soft surface until it's flush with the board. Don't use too much force, or you will break the board or scratch the matrix.

Solder all of the pins on the back side. Start with the side ones, and then do the middle.

Use wire cutters to cut off the excess pins, as flat against the board as possible.

If you have a small battery (I used 110mAh), you can add it on the back of the board.

Cut the battery wires to about 0.5cm. Be careful to cut them one at a time, otherwise your cutters will short the battery and make sparks fly.

Carefully remove the insulation and tin the ends of the wires. Be careful to not short them.

Solder the wires to the holes marked + (red wire) and - (black wire).