Short recipe:
1 gallon baggie with 3 T baking soda and put into it **WITHOUT SPILLING ANY** a glass jar or measuring cup with 2 C white vinegar.
You need a 2-3 ft piece of vinyl hose from the hardware store to do it without upsetting the animal.
Put animal (up to 2#) in a 1 gallon container. If you change the volume of the container the concentration will change and you may have an ineffective mix- or have it too strong, which would cause a short period of severe discomfort for the poor thing.
To knock the animal out, put the tubing into the bag, and the other end into the jar or container containing the animal. Holding the bag tight around the tubing squeeze all of the air out of the bag, but leave a small gap around the plastic or whatever holds the tube into the container. This will prevent a huge crush of pressure from hurting the animal right as the mix occurs, and since the CO2 sinks, it will be normal air escaping. Pour 1c. of the vinegar into the bag while holding it tight around the tube. If fizz starts to go into the hose, lower the bag so that it doesn't wet the chamber.
Wait until animal is unconscious, then pour the remaining vinegar into the bag and allow it to remain for several minutes.