The easiest way is to take pictures of leaves or bricks or some other stuff.
The more chaotic the image, the better. Also, the image has to be small.
One thing that I do sometime is make a picture of a forest, take out of it just the trees branches and leaves and shrink the image a bit.
Anyway, I haven’t experimented that much with patterns. Also not all patterns will work with a certain mask.
If the generated stereogram is messy, try with a new pattern.
Uploading a picture of your self small will work in some cases, but it will mess a bit the image.
But most of the time the hidden image can be seen, although not that clear enough.
Anyway, enjoy, experiment with patterns, have fun!