The app integrates directly with WhatsApp.
Go to settings (upper left squared icon) and tap on the WhatsApp option. Follow the instruction on that screen choosing the emojis you want to use. WhatsApp will open automatically on a screen with a button to Save the stickers (or, if you already saved them to WhatsApp before, to Update or Delete them).
A sticker icon will be visible next to the box where you type your text. Tap on it and instead of your keyboard you will see an area to select either GIFs or Stickers. You can alternate between GIFs and Stickers using the two icons at the bottom of the screen. Select Stickers. The pack you just saved will appear as one of the small icons at the top of this section. Just tap on a sticker to use it. You can delete stickers either following the same steps from within the app, or within Whatsapp itself.