To add stickers to your iMessages from Star Wars Sticker packs:

1. Launch Messages and tap the "App Drawer" icon
2. Tap the Sticker Pack icon you wish to use (These aren't Sticker Packs you're looking for?)
3. Swipe left and right to browse through available stickers in the pack
4. To add stickers to your conversation, tap to send automatically, or touch and drag the sticker to a reply or image in the message. In this conversation, an adventurous Kitty is sporting a Resistance Pilot helmet sticker.