Your nendoroids deserve a warm, cozy night by the fire this Christmas.

Materials:

Miniature wooden fireplace
Adafruit Pro Trinket 5V*
Mini PCB Neopixel 5 pack
30 awg silicone wire
Micro USB cable (ideally a right angle USB cable)
Solder
Hot glue
Quilt batting scraps
Decorative ribbon

Tools:

Soldering iron
Hot glue gun
Alligator clips
Power drill + 1/8" drill bit
Needle nose pliers
Wire cutters
Flush cutters

*Some notes on microcontroller selection: I wanted something with a built in USB port. I was originally planning to use a Trinket M0 but FastLED does not work with that board yet. Also, the original Trinket (attiny85) was too slow for the animation speed I wanted.

Step 1: Solder LEDs

Break the 5 LED strip into sections of 3 and 2. They're not actually electrically connected so you'll need to solder them together. This is a very tricky soldering job. You can use the header pins included with the Pro Trinket to solder the pins together.

Tin all the pads
Use needle nose pliers to pull a header pin out of the black plastic.
Trim off about 1/3 of the length with your flush cutters.
Place the pin between two pads. Hold down with needle nose pliers or a fingernail.
Press soldering iron down on one end of the pin until the solder on the pad melts onto it and is holding it down a little.
Now solder the other side of the pin - add some solder.
Go back to the first side and add some more solder.

For a demo, I have a video on my twitter.

Once you've soldered the LEDs together on the boards, add wires between the sections. Make sure to follow the data in -> data out direction!

Measure out the right length by placing the LEDs inside the fireplace where you want them. You'll want to make the data and ground wires between sections slightly longer since they have to go a longer distance, from the bottom front to the top on the 2-LED section when placed in the fireplace.

Also solder wires on at the front of the 3-LED section - long enough to reach through to the back of the fireplace.

Step 2: Upload Code to Microcontroller

If you don't already have it set up, follow Adafruit's tutorial for installing drivers and preparing the Arduino IDE for use with the Pro Trinket.

The sketch is attached. I used FastLED to generate the animations, so please make sure you have installed the FastLED library.

Thanks to Erin St Blaine whose code I based this off of.

minifireplace.ino
Download

Step 3: Test LEDs

I recommend trying out your LEDs to make sure everything's soldered properly before proceeding.

Connections:

Data -> pin 3
5V -> BUS (USB power)
GND -> GND

You can use alligator clips. Clip onto the wires going into the LEDs and carefully clip onto the board. I recommend using one of the GND pins farther from BUS so you don't accidentally short anything.

Step 4: Prep Fireplace

Using a power drill and a 1/8" bit, drill a hole in the back of the fireplace so that you can feed the wires through to the back.

Step 5: Glue LEDs in Place

Feed the wires through to the back of the fireplace, and then use hot glue to attach the LEDs to the wooden fireplace. Hot glue sticks pretty well, but it can be peeled off if you make a mistake.

Step 6: Solder LEDs to Microcontroller

Now that the LEDs are in place and the wires are run through to the back, solder the wires to the microcontroller.

Red: 5V -> BUS (USB power)
Black: GND -> GND
Yellow: Data -> pin 3

Trim excess wire with flush cutters. Hot glue the microcontroller board to the back of the fireplace.

Step 7: Add Diffusion

Cut out two small pieces of quilt batting and tuck them into the fireplace. You could use something else for diffusion if you like - maybe paper/cardstock or acrylic cut into the shape of a fire.

Step 8: Decorate

You can use some holiday ribbon to spruce up the fireplace if you like. I cut some green tinsel ribbon to make a garland and hot glued it on to the fireplace. I also tied a small bow and hot glued that on as well.