Super easy, you can do this a few different ways.
First you can use glo sticks, cut the ends and shake the liquid inside of the mason jar, put lid on and then shake it up all over so the liquid covers the inside. Very easy and cheap that way. I suggest parents do this with goggles and gloves because the inside of glo sticks can irritate you if it gets on skin or the eyes, this is not my favorite choice but for adults only its an easy and cheap way.
Second way is to buy glo in the dark paint, that is very easy but can cost a little more $, you just paint the inside of the jar and add little dots to make them look like the Fireflies. This is my favorite to do with the kids since its safer than the glo sticks that you have to break.
Third you can buy glo in the dark nail polish, do this the same way you would with the glo in the dark paint. I like this way because I always have finger nail polish of all colors and like to add purple into the mixture, it looks so pretty!!
Doesn't this just SCREAM Summer!!!!? I love this!! Reminds me so much of being a kid and running through the yard catching them. We would remove the glo and rub it on our faces, yes I know we were bad kids!! I do not encourage my kids to do that just so you know ;)
This is so fun to do for a cookout or those late night talks around a campfire and have these hanging in the trees. Also these are VERY good to use at night lights for the kids in their bedrooms. I just love this time of year!! Why is it that the Summer brings out the sentimental part of me? This time of year and Christmas, I guess because as kids we ALL have good memories of the Summer and the Holidays. I hope you all enjoy. If you make them please send me a picture so I can share these.