In order to make the image file for stickers, you need to create a transparent background in Photoshop or a similar image editing software, and add the stickers to it while following some basic guidelines.
Image requirements can be found here, and stickers are within the sheet called "Prints.”
Let's use the variant DieCutStickers-55x55-5Pack as an example.
If you look at the image requirements for this, you will see that required image size is 1575 x 1575 pixels. The file should have a resolution of 300 pixels per inch and each size has a bleed of 25 pixels.
That means you should make a transparent layer in Photoshop sized to 1575 x 1575 pixels and position your solid color stickers on it, but making sure they are not positioned completely next to the edge since 25 pixels will be trimmed from each side during the production process.
You can use this tutorial to check how you can create transparent layer and manipulate the images around its surface.
The manufacturer will add a 1/8" white border around each sticker, which is why it is important for them to have the solid edge.The final product is sheet in shape of a rectangle with stickers and the excess parts around them. Something like this:
Please note that your sticker designs must be a solid color, and everything that needs to be cut (excess parts) must be fully transparent.
Keep in mind that in case you wish to print several smaller stickers instead of one big, it is very important to space them correctly - there should be at least 1/4" space between the stickers. Each sheet is not limited to a number of stickers, you get as many stickers as you can make fit to a required image size with the border.