Hello I followed this tutorial to create a bottle as an OBJ in Photoshop:
It works well except when I try to apply a decal it appears to be inside out. Attached is a screenshot of my PS and the dimensions rendering.
you built the bottle wrong... ask your teacher to show you the correct way to do it or have a read of these notes
Looks like you may have either co-planar geometry (you have two copies of the same thing in the same location), or your UVs are flipped inside out.
The instructions in the first attempt of that video weren't very clear - so the author made a new one: https://www.youtube.com/watch?v=c0M0iQ45nJ8
Maybe that'll help your build. 🙂