It’s not possible to make clothes in Roblox.
Roblox’s clothing template system enables you to purchase clothes from the catalog with your Robux or generate clothes using scripting, but there is no interface for editing the look of a piece of clothing that exists within Roblox. As a result, if you’re looking to design clothing for your character in game, we recommend using one of many free 3D modelling programs. The most popular ones are CubicStones and Blender. You can then export the modelled 3D model into an .XML file which can be imported into our developer tool Space Pets without any further conversion required (other than at import time) and transformed into an outfit that will work on Roblox.