How to make a necklace

Hi. How to model a necklace depends on the shape of it. As for adapting, it’s pretty much the same as any other mesh-based cas object.
You should start by looking through the catalogue to find a similar necklace. You will notice that they never go too close to the breasts and it’s best if yours doesn’t either (it will distort with breast morphs, it’s inevitable).
Once you will find a similar one, export the blend, open and study it.
You will see two meshgroups: one for the necklace and one for the cube inside the body. This is done for the necklace to properly interact with tops and you should leave it in the scene, replace only the necklace itself.
You would see that the necklace is affected by the same bones as the body underneath it. Same goes for uv_1, so both should be transferred from the nude top instead of another necklace.
You will also see that it is vertex painted with 007F00 - morphs just like the body underneath but does not highlight when you hover over it in cas
And if course it’s uvs in uv_0 are in the space meant for necklaces.
Once you adapt your necklace like that, make sure to assign the same cut as the original had.
That’s about it.

Well, I would need to see the files to know for sure but you either made it too long or replaced the cube instead of the necklace