Gather your ingredieents and supplies together.
Pregrease your pan and spatula with either butter or a non-stick cooking spray.
Melt your marshmallows and butter together - this can be done via microwave (instructions below) or via stoveop (instructions in post above).
Dump 4 tbs of butter and 16 oz of marshmallows into a bowl that can be used in the microwave.
Heat butter and marshmallows in microwave for 30 - 60 seconds.
Take them out and stir them together well.
Place back in the microwave and heat for 30 seconds, then stir.
Continue heating and stirring in 30 second intervals until the marshmallows are fully melted
Separate your melted marshmallows into two bowls of even amounts. (alternatively you could separate the butter into 2 tbs and marshmallows into 8 oz and heat each separately).
Add green food coloring to one half of the marshmallows and stir well.
Add 3 cups of cereal to each bowl of marshmallows and fold together well. You should now have half a batch of regular rice krispies and half that are green.
Take your pan and press your treats into the pan, one half for each color.
Set the pan aside to cool for around 30 minutes.
Once cool, use your cookie cutter to cut the green treats - only cut the tail part of the fish, not the whole body.
Then, cut small rectangles from your plain rice krispie treat side (this will be your mermaid body).
Once you have your tails and body's cut, you can take one body and one tail and press them together to form your mermaid.
Add sprinkles or other decorations as desired.