If you don't have a stencil you can always fold your paper in half and draw half of a butterfly but it won't work as we'll. you'll see what I mean later. You can use something similar to my stencil
Trace your butterfly stencil. I personally don't like to use the circular head part of my butterfly.
You can colour it now or wait until it is taped.
Stick a piece of scotch tape facing up or down depending on whether you decorated or not
Stick the wings on the body you've just taped. It should lay flat in your hands... Now to make it a little bit more realistic you can add decorations.
Now close one half of the wing then the other. It might take a couple of times for it to stay
Now repeat on the other side and... Voila you are done