Most of what we know is from Pompeii and Herculaneum. As they used frescoes (pigment applied to wet plaster) they did not survive well. While they sometimes had sections of plaster slightly raised, they mostly used perspective, light, and shadow to create the 3D effect.

There are four main styles (with overlap as fashions changed):
1-  Incrustation (looks like blocks of different stones)
2- Architectural (adding architectural elements like columns)
3- Ornamental (increase in decorative elements)
4- Intricate (many fine details and decoration, often on white background)

You're in luck, I happen to have the Ppt handy so here, in order, are some examples of each.

https://media.spinster.xyz/a7c105a2bd0780f63369a4095a61ab1f58115618fe8a47a0505af50a776f9023.png

https://media.spinster.xyz/1d6fa1ce4a39f2adf546574b4aa018c08500a4b030501176d3c2b63c44923dbd.png

https://media.spinster.xyz/25cc1f4a22f1e613901bea2d3b4cc0b90785754d04545d402e56ee2b11f8c4ce.png

https://media.spinster.xyz/e182b6b57ad64ebee86514c4e5eb563314192d1c3d6053b393894c9b0feadf6c.png