I love these! They really help with anxiety, and they're a fun project:

Glitter jars: How to make your own calm down jar or bottle - goodtoknow

Calm Bottle (aka Glitter Jar, aka Mind... - Creative Social Worker

The way I do it is I fill a jar or bottle about 1/4 with clear Elmer's glue (not the white kind). Then I fill it up to about 3/4 full with water. I swirl it around to mix the water and glue, and then I add whatever glitter I want. Don't add too much, or you won't be able to see the glitter well if it's too dense. Swirl it around to see how it looks. Then once you've got the glitter how you want it, top it off with water, and screw the lid on tight.