I am pretty new to Blender but with tutorials I came at least this far.
I made a mesh blanket over a mesh bench. I used cloth simulation to make the blanket a nice fit and at the frame I liked I used Apply under the Moderfiers tab in Properties ,to make the cloth a mesh.
This is the front of the blanket and then the back with Solidify applied:
But when I try to upload it into my virtual world, the back is still blank: