Once you activate Bluetooth on your iDevice, Launch ProCreate.
Create a new blank canvas.
The canvas should appear with all of the interface elements.
Look to the upper left corner of the Procreate app for a wrench-like symbol.
Tap on that symbol.
It opens a window where you can pick your type of stylus and then below that is a large circular dot that you tap and hold the stylus to use to pair the stylus to the Procreate app.
FYI,
Most apps that take advantage of stylii will have some settting from within the app, itself, that you will need to use to pair a stylus to the app.
Every app that uses a stylus input pairs stylii using slightly different methods or settings to do so.