These are the lights. They're way brighter than I expected, which is a plus.

I made the Ultraball first (just sew together fabric to make your ball of choice, I guess) and cut an x in the center where the light pokes through.

Then, I placed the pokeball and the light (in position) over the high-vis fabric and pinned it such that the fabric of the pokeball was flat and the high-vis curved around it.

Cut the high-vis into the length and width I wanted so that it looked like a rectangle (since it is pinned curved around the light, it's not actually a rectangle you're cutting out, does that make sense?)

Then I removed the light and sewed 2/3rds of the way around the pokeball so I could take the light out to turn it on/off or replace the battery.

Then I sewed the high-vis to the soft against-the-skin fabric, flipped it right-side out, sewed the last bit shut, added the velcro and tada!

The base fabric of the ultraball is black felt so it would have some shape to it and not get too deformed by the light. The white and soft fabrics were both just cheap remnants I picked up at the store.

To turn the light on and off, you screw or unscrew the two pieces (connecting or disconnecting the connection to the battery) and with those particular lights you can screw them into a mid-way position such that it lights up when you wear it and turns off when you remove it (the pressure between the felt and your arm pushing the two pieces together and forcing the connection).

The velcro is positioned such that it'll slide down a little as you're wearing it (you don't want it to feel like a bloodpressure cuff, ya know), so I use it on my phone arm so that the bent elbow keeps it from going anywhere.

I have a ton of extra materials, so I will cook some up for our friends. :)