All stickers on the Redbubble marketplace are kiss-cut, which means the edge of each sticker is cut by a sharp metal die or laser but the cut does not penetrate the sticker’s backing so they arrive on a small sheet.
Stickers are cut to the exact shape of your design, plus a 1/8 inch (3.2mm) white/transparent border, so they can be easily peeled off from the sheet.
In order to get your sticker to be cut to the shape you want, your artwork needs to have a transparent background and the file must be saved in PNG format. If you feel unsure of how to create a transparent background, you can check out this blog post for more nitty-gritty information!
Once you have your transparent background, you’ll need to check for stray pixels. These might be caught by the machine and be cut around as well, so the sticker can end up having extra white/transparent space that doesn’t belong there.
A trick to eliminate all of these pixels is to create an extra layer under your artwork and fill it with an intense color that is not present in your design. This way, it’ll be easier for you to spot the pixels and erase them.