https://sims4studio.com/thread/10/tutorial-using-sims-studio-accessory

Introduction

This tutorial will show you how to clone an EA accessory, switch out the mesh so that your new mesh takes its place, prepare your mesh for import back into the .package, and then get the .package ready for the game. This tutorial is meant for those who already know how to make a new accessory mesh and texture and will not show you how to create a new mesh. Instead the focus is on the Sims 4 specific elements of the process.

What you will need

Sims 4 Studio

Blender 2.7.0

Your fully mapped accessory mesh - note that your map should be in the same place on the image that EA's item is mapped in.

Textures for your accessory mesh

Instructions

  1. Open Sims 4 Studio and enter your creator name if you have not already done so. Put a dot in the Create 3D mesh radio button.

  2. Click the CAS button.

  1. Use the filter boxes to limit the thumbnail list to the kind of item you want to make. This tutorial will use an adult female earring as the example item.

  2. Click on the item you want to clone. If you would like all the swatches to be included in your .package hold down the shift key as you click. By default Studio will only include one swatch in a new mesh .package otherwise.

  3. Click the Next button. Name and save your .package in your project folder.

  1. Studio will open your new .package and the item you cloned will be displayed on the model in the model viewer. Click the Meshes tab to switch to the Meshes section.

  1. This is the Meshes list box. By default it will show you LOD 0. If there is more than a single LOD in the .package this will contain a drop down list of all the LODs. LOD stands for Level of Detail and a CAS LOD consists of meshes, references to the bump image, and other data relevant to the item's behavior in the game. LOD 0 contains the meshes players see in CAS and when viewing the item close up in the game. The higher the LOD number the further away the player will be viewing the Sim when that LOD's mesh is displayed.