Nobody has mentioned it yet, but if you're using a program like Gimp, Illustrator, or Photoshop, you should use layers. Make a new layer for your background. This is the shirt color at (https://lilpeepmerch.net/product-category/shirt/) I'm designing for—almost it's always black, but I use a dark grey for visibility. The background layer should be the final and lowest layer. When you're finished designing, simply hide the background layer, and you should get a transparent background when you save as a.png.
Nobody has mentioned it yet, but if you're using a program like Gimp, Illustrator, or Photoshop, you should use layers. Make a new layer for your background. This is the shirt color at (https://lilpeepmerch.net/product-category/shirt/) I'm designing for—almost it's always black, but I use a dark grey for visibility. The background layer should be the final and lowest layer. When you're finished designing, simply hide the background layer, and you should get a transparent background when you save as a.png.