Import (File > Import) the stereogram imagine and position the colored symbols on top as shown.
(You can delete the background rectangle that was behind the symbols).

We'll add transparency to the symbols to make them blend better.

Select the top row of symbols, then select the Transparency Tool. Leave the type as Mix and Move the slider on the Infobar to 30%

Select the middle row and apply a Bleach transparency, 0%

Select the bottom row and add a Mix, transparency, 30%.