On a piece of paper, draw a simple butterfly shape such as this one using your pencil or pen.
You can make it any size you want.
Dip your brush into water and then into your desired watercolor paint color, and start painting the wings.
DON'T WORRY IF YOU GO OUTSIDE THE LINES. IT WON'T MATTER.
Color in the butterfly's body black.
Start decorating your butterfly with swooshes and dots and whatever else you want on the wings.
I used a #6 pointy round brush for this part.
This is what mine looked like at the end of painting. Not my best work, trust me.
Cut out your butterfly.
This is where you get to fix mistakes like painting over the lines, and eliminating the pencil lines.
Fold your butterfly's wings against its body to create the 3D effect.
If you don't have double sided tape, you can use regular tape by folding it on itself and sticking it on the back of your butterfly.
Now attack your door with your butterflies.
TA DA! Now you have pretty 3D butterflies. Enjoy!