Last year we asked our sons what they wanted to be for Halloween and they both wanted to be fireflies so of course that was what they were going to be. It went without saying that they had to light up so my wife started working on the costume and I worked on the lights. Of course there wasn't much time until Halloween so it had to be pretty quick and easy.
Step 1: Light
For each costume I started with a cheap high powered LED flashlight and took out the LED assembly.
Step 2: Batteries
Then I took 2 AA batteries and put a piece of jumper wire at one end with some foam tape to hold it in place. On the the other end is the wire that goes to the battery connector and to the LED assembly. I soldered the wire to the LED assembly leads.
Step 3: Switch
To make them able to light the lights when they wanted to I soldered a tiny switch and put heat shrink on it. I used the battery connector so the switch could be ran inside the sleeve of the costume and be removed from the light and the rest of the costume without having to pull the wire and switch out.
Step 4: Firefly
Here it is assembled. I don't have pictures of the rest of the costume parts but for the firefly light I cut out a sort of oblong half circle shape (see the drawing below) from thick styrofoam and cut a hole in the middle for the light. The hole I cut was just right and didn't need glue to hold it but I think hot glue would work fine. The battery pack was stuck to the styrofoam with 2 sided foam tape. My wife sewed a bag from yellow fabric and to make it keep it's shape she put fabric interfacing on the inside.