Instructions
Open a new transparent document in an image editing program such as Photoshop or GIMP. Save the file to your desktop as a .PNG, the only file format that allows for transparent backgrounds. Most graphic editing programs allow you to set a new document's background to transparent.
Launch Roblox and click "My Wardrobe" at the top. Click "Browse" from the top and click on your transparent .PNG file.
Click "Create Shirt." Your new transparent shirt appears in your wardrobe.