Here's the steps I know from watching UGC showing their process of making stuff (from memory) :
1. Get blender and import the roblox model of your choice (Robloz package)
2. Add another layer and use that to make the item
3. Align the item with the roblox package
4. Save and then import your item model to Roblox studio and realign it incase something happend
5. Upload and the rest is a mystery since UGc creators arent aloud to show the process on how they upload stuff onto catalog
Btw you need to be in the UGC program or be a star creator to uplaod your item onto roblox catalog but if you know another player who's apart of the UGC program you can ask them to uplaod your item (they'll give you some of the profits and prpbably keep some for themselves as a fee for uploading your item for you)
Wondering how to be noticed?
1. Train alot using your prefered 3D moddeling software and start uploading your work on social media to grain attraction from players
2. Have a proffesional thing that shows off you skill (I forgot what their called but I think it's called a profile idk)
If your lucky or you think you have gained a good amount of influence you can sign up to be apart of the UGC program though just know it might take a few tries
Anyway it's like 1 am for me there's prpvably alot of things wrong so uh sorry :(
