Open your browser and go to EasyStereogramBuilder website. Click on the "Build your own" button on top of the page.
Click on the empty box with the word "Pattern" written inside, then choose a pattern you want to use, below the box. You can also click on the "Upload your own pattern image" button to use your own image. Click the "Apply changes" button.
Click on the "Mask" box to choose an image from below, for your 3D stereogram image. You can use your own image by clicking on the "Upload your own mask image." Click the "Apply changes" button.
Click on the third box in the row, "Stereogram." This renders your stereogram, which takes only few seconds. The "Congratulations" box appears with your stereogram image thumbnail.
Click on the image thumbnail to enlarge it and look for your hidden image.