Hello!

Is there a tutorial that I can follow to convert wearable accessory jewelries into deco objects?

Thanks for your help in advance!

There aren't any tutorials on how to do that spesifically.

You can extract the mesh via a meshing program (you'll have to use a meshing program anyway to reposition the earrings - Milkshape or Blender is what you'll usually find tutorials for nowadays), and extract the texture via SimPE (if it's originally a TS2 item), then use a regular object tutorial to make the earrings into an object.

You might need additional tutorials like how to add proper shadows, or how to fix the texture if it's an alpha mesh (see-through/cutout).

There's a list of tutorials here (object meshing further down a bit).
https://sims2tutorials.tumblr.com/meshing
You'll need GUIDs, there's a topic below on how to make those, since the SimPE site is down.
https://modthesims.info/showthread.php?t=638392

Just a tip at the end - for small deco items like jewelry, to not get thumbnails that shows the item from very far away, and placeable on all shelves, I prefer cloning the wooden sculpture of a man (the kind you'd use for drawing poses with). These things are possible to edit, but in the beginning it's easier if you don't have to worry too much.

Thank you so much for your helpful and guiding response!