how do you start designing roblox clothes?

template is 585 x 559, save it as a png. if your shirt looks stretched once it's on, it's almost always the canvas size being off, not your art.

Grab the official shirt/pants templates from the Roblox creator docs and draw over them. Photopea is fine if you don't want to pay for Photoshop. Keep the image the exact template size and only paint inside the panels, or it'll sit wrong on the avatar. Test it on a dummy in Studio before you upload, because uploading costs Robux. And study how the good shirts shade the folds and seams. That shading is what makes clothes look 3D, much more than the design itself.