First download and install Stereogram maker from here.
You can use elefont to create a suitable 3D DXF file to use in your stereogram. Or you can use any image as elevation map. Download elefont from here.
It should be easy to use elefont to create a DXF from your favorite font. Save your work as DXF.
Open your DXF in Stereogram Maker
Rotate and scale until it looks good. Then create an elevation map by clicking here:
This is what it should like:
You can also create your elevation map with GIMP or other software.
I used above image as input for pattern / texture. Drag and drop on Stereogram Maker. When asked answer "As Pattern"
You can now create your autostereogram... Just click on create stereogram.
Here is the resulting image:
Find more info here : https://en.wikipedia.org/wiki/Autostereogram