Glad to have you on board

To get a 3D look on your character, you will need to create vertices inside the mesh that you can manipulate.
See more here: Meshes - Spine User Guide: Deformation

I've created a small example project that shows how to do create a cube like this:

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Download: cube.zip

A few more vertices will allow you to do something like this:

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Try practicing faking 3D with simple "objects" first.

I hope this helps

Little tip from myself.
You have to change point of view, try to look on the 3D effect not as a whole but as scale, rotate, move etc. Try to see which element and how would be moved by scaling, rotating, translate tools which you can use in 2D. In fact using those tools, parts, you can build illusion which well folded gives the 3D effect It really helped me when I changed perspective.

Hi Shiu, can you tell me how to do it step by step? thank you