Which shirt template do serious roblox clothing designers use?

on top of that, work at double the size in your editor and only scale down to 585x559 when you export. shading done at the final size gets muddy fast. and export as png, jpg chews up the edges.

Everyone uses the same official 585x559 template. Being serious about it isn't about the template, it's about how you shade it. Most people draw in Photoshop or Photopea with a shading layer on top and export a transparent PNG at exactly that size.