- Hold Option / ALT & drag to duplicate so you have 3 of the image.
- Make darker. Place 1 on top & 1 below. Select both.
- Go to Object > Blend > Blend Options & Set “Specified Steps” to 200.
- Press OPTION / ALT + COMMAND / CTRL + B
- Object > Expand then Expand Appearance
- Use Pathfinder > Unite
- Add a stroke!