Here's a sardine before it's butterflied.

Cut off the head.

Cut (at an angle) the bottom portion of the the fish from the belly to the head (see the diagram in Step 10). Then slice the bottom of the fish all the way across.

Using your thumb, open up the belly starting near the head and move toward the tail.

Use both hands to remove the spine with your thumb and index finger (I could only use one hand in the photo), starting at the head and moving toward the tail.

After removing the spine up to the tail...

Break it off with your hands.

If the skin near the head of the fish bothers you, cut it off to round out the shape.

Rinse under water, then blot away the excess moisture.

The drawing shows where to cut the fish in Step 3.

Here's a recipe for Kabayaki Sardines.