They couldn't be easier to make...
2 cups of baking soda
1-2 cups of water
Several drops of essential oil
Heat the water to boiling and add the oil. Mix the liquid slowly into a bowl containing the baking soda. Add only enough liquid to make a spreadable paste. I needed about 1 cup of water. The more water you mix in...the longer the discs take to dry and harden.
Spoon the paste into small containers. I used aluminum muffin tins but I think that silicone molds would work great. Don't do anything to the tins (like greasing them).
Let the soda completely dry. This may take several days or just half a day depending on the humidity in your air. Tap the discs out of the tins and store in jars or bags until you need them.
I popped one in the bottom of my garbage can and so far, it is keeping the can smelling pretty fresh. I read that these work wonders in diaper pails, cars, pantries and refrigerators too.
Some people who made them put them in their wash after using them elsewhere. I don't think I would do this. They are so cheap and I'm not sure if I want them in my wash after using them someplace like a diaper pail.
Try them. I think they are great little things to have on hand.