I had 6 lights remaining from a strand of programmable rgb LEDs left from making a white fur scoodie and I've been trying to come up with a project to use them. I also had a few leather belts laying around that I rarely use and decided to try this out.

I realized how similar it was to the NeoPixel Punk Collar project so compare to figure out what might work best for you! This is far less soldering and wires!

I put this together in an evening (still in my work clothes!)

You should be comfortable with:

Using a drill
Soldering
Uploading a sketch to a microcontroller

Step 1: Gather Your Materials!

I had 6 ws2801 lights around; these lights are nicely diffused and fit in a 1/2" hole. The number of lights you use and your belt size may differ but the project is basically the same. My light strand reached just around my hips when centered in the back. You will want to use a belt with some room since you won't be wearing it tight, too. I went with a CR2032 coin cell holder with a built in switch since it was easy. I haven't done any calculations on the lifespan but you may want to consider a different battery pack if you want.

Leftover lights - I used 6 x flat ws2801 lights from adafruit
Belt
Battery/Battery holder
Trinket
Alligator clips
Soldering stuff - iron, solder, etc.
Drill & bits - you will need a 1/2" bit plus something smaller to make pilot holes
Hot glue gun & glue or something fancier

Step 2: Put Some Holes in Your Belt

I played with the positioning a little while. Initially, I started in the back center and put a dot in the spot I wanted the first hole. I figured that I would just do one at a time and wing it, not really taking the flexibility or the existing holes of the belt into consideration. In hindsight, I should have started with the tightest (last) belt hole already on the belt since I pretty much ended up there and it was an existing hole. In addition, I should've just measured and marked out my holes but it all worked out. Since the belt wraps around your body, I found that while some of the holes were perfectly distanced while laying flat, there was excess wire when the belt was on, whereas the one that was a little too far away was perfect. It all worked out. Anyways, you may want to do one hole at a time or all at once, up to you.

Mark your holes - I used a fancy silver sharpie
Drill 'pilot' holes using the small drill bit - I held down the belt with my hands and feet*
Drill the pilot holes with the larger 1/2" bit
Clean up your holes using snips or scissors
Pop your lights into the holes. See the note for how to orient. I will have the power and trinket on the belt loop side so my arrows point toward the side with my belt holes.

Note: the ws2801 lights have arrows pointing in one direction, which is the travel of data. Since we will be powering this all together, the power and your trinket should go on the side where the arrow originates. Since the power and trinket will go on one side, orient the direction of your lights so that you have space for these things.

For example:

Power - Trinket - Lights with arrows ->

* Be safe!

Step 3: Connect Your MC and Battery Source

It's always good to test your circuit first using alligator clips so do that! Go upload some blink test code or the recommended code or whatever you want now, too. You can change the Yellow and Green pins but be sure to change them in your code as well. Once you've done this you can solder it all up. I did a pretty crappy job soldering so I didn't take pictures and quickly covered it all up with hot glue. I was so embarrassed, I almost covered up the restart button on the trinket!

Here's the code I'm using as uploaded to my codebender page: https://codebender.cc/sketch:170040

Use alligator clips for steps 2-5:Connect the Yellow Wire on the lights to #3 on the trinket
Connect the Green Wire on the lights to #1 on the trinket
Connect the Red Wire on the lights to the 5V on the trinket
Connect the Blue Wire on the lights to the GND on the trinket
Connect your trinket and upload the code to test your circuit
Break out the soldering stuff
Twist the Blue Wire on the lights to the ground wire (BLACK) from the battery pack
Solder that connected wire (GND wires) to the GND on the trinket
Solder the power wire (RED) from the battery pack (make sure its OFF!) to the "BAT+" on the trinket
Solder the Yellow, Green, and Red wires from the lights per steps 2-4 above
Attach the Trinket to the Battery pack and place inside the belt in a comfortable spot