Whatsapp stickers are nothing but images in PNG format with no background transformed into stickers just like using sprites in games or other similar applications.
You can easily make Whatsapp Stickers following below steps
1. Select images in any format to convert as stickers
2. Open images in Photoshop or similar applications
3. Select and erase unwanted areas from the images
4. Save images in PNG format with preferred size
5. Install the app Personal Stickers for Whatsapp from Android Playstore
6. Open the app and select the edited PNG images from the list. You need atleast 3 images.
7. Convert the images to Whatsapp stickers. Done
Now go to Whatsapp and click on the smiley icon. Your newly created Whatsapp stickers will be there on the list. Try sending one.