"Hewston, we have a problem: Who ate all the cheese?"
Where to find Hewston after installing the sticker pack...
Open up a message and tap the ">" button
Then tap the App button (it looks like an A)
Next, look in the lower left part of your screen for the button with the 4 circles and tap this to open your iMessage sticker app drawer
Hewston should pop up wearing 3D glasses. Tap the icon to start using the stickers. Enjoy!