This instructable will show the many steps to completion.
Step 1: Download and install a stereogram maker.
(I am not the creator, just a user)
I installed it and opened it up.
Step 2: Choose your image
The next few steps will be preparing the image to create a nice, rounded end image.
Step 3: Editing the image
This will make the stereogram maker give it depth when it processes it.
I then used the select tool to select the area outside of layer 1, popped back to layer 2 and filled the selection with black.