Aleš Lombergar

Assuming you are not talking about holograms that are made with lasers, there is a much older technique, that is done in clear plastic rather than actual glass, That is done with drills and burrs from the back by making a hole the shape you want to see in the front. it is a bit tricky to get the hang of but not real difficult once you do.

If you can get the plastic and a flexible shaft tool and some samples to play with, you could be doing some nice work fairly quickly, I did it in shop class in Jr Hight school.

Getting a 3d object in real glass is a much trickier proposition as the piece has to be put together first in a lamp working glass technique and then at the furnace glass is put around it. there was a technique where a very special clay was made that was compatible with the glass and then objects made of that clay were put into the molten glass. These were called “Sulphides” though I don’t know that any sulfur was ever involved..

Still another way a similar effect was done is to make a clear resin in a mold and drop actual 3d objects in them, a favorite is to do that with a dandelion head.. This method and carving the plastic from the back are the easiest to try at home.