Its easy to create whatsapp stickers now, First you can download the demo from github which is provided by whatsapp.
You can create your own stickers just follow below link.
Sticker files must be in WebP format, you can easily convert a png file to WebP from AndroidStudio itself.
Add the image file (png format 512*512 size) in to the asset folder under a folder given. The folder name must be number alphabet etc.
Convert the image to WebP format.
For this step just add the identifier (folder name) and the image name (which is logicchip.webp) in contents.json.
its all done now just install the application.