How to make 3d costume

how to publish accessory like this (i have 3d model) Egg Cat Costume - Roblox

As of right now, you cannot upload UGC items unless you’re part of the UGC Creator Program - where people are selected specifically or based off of applications which are not currently available (I believe).

In the Developer Roadmap, it’s stated that at some point in “Early 2024”, UGC creation would be opened to ID-verified Premium users. But this has not happened as of yet.

You can upload avatar accessories as long as you meet the criteria. This announcement has an overview of the requirements, as well as other information to help you get started uploading. If you need additional help, I’d suggest searching for tutorials related to the specifics of what you’re uploading. Just make sure to check out the guidelines and policies for UGC items before uploading your item.

I hope this helped, and good luck on your UGC journey!

i don’t need to upload for sale, i just need accessory in roblox studio

This is called layered clothing and there are a few tutorials as well as official Roblox documentation that teaches you how. It even works without you having to upload it to the UGC program.