Answers to the usual questions, and how to reach the developer.
Open a chat in Messages, tap + next to the text field, then More, and choose Stickerit. Tap a sticker to send it, or hold it and drag it onto a message.
WhatsApp accepts packs of 3 to 30 stickers. Add a sticker or two to the pack, then send it again.
Add, change, delete or reorder stickers in the app, open the pack and tap Send pack → Telegram again. The same Telegram pack is updated and keeps its link.
Open the sticker, tap Cut-out, then Refine cut-out: paint with Keep or Erase, or draw around it with the Scissors. Pinch to zoom in for fine edges.
Go to Settings in the app and tap Restore purchase, signed in with the same Apple Account.
Something else? Email vladimir.matusevic@gmail.com — please mention your iPhone model and iOS version.