It should automatically detect the sticker images in the Download folder and sort them into different sticker packs according to the available subfolders.
Add custom stickers to WhatsApp on Android
Similarly, having a non-transparent background on a sticker may impact its aesthetic value. For example, using an image that isn’t a “square” will likely distort your sticker while being imported into WhatsApp. Note: The method we’re going to follow tries to automatically take care of most of WhatsApp’s sticker requirements although, for best results, it is recommended that you follow these requirements as much as possible.