If you don't know the answer, the question isn't dumb .
The secret is a lot of handy work, by selecting layer by layer, and copy to the other side, 95% of the time you then have to flip that decal to get the unmirrored.
And yes, that is a pain in the an... department.🤬
There's 5 tabs after you finish placing a decal: Set Projection Area, Layer Controls, Decal Colour, Edit Decal and Finish Editing Layer. The copy and paste you're looking for is the 4th option in the Layer Controls tab called "Duplicate on Reverse"