How do I change the settings for follower and subscriber notifications?

Open the XSplit Gamecaster Overlay by pressing [Ctrl] + [Tab] (or the hotkey you assigned to it) and hover over the Twitch logo and click on the follower icon:

This will open the Twitch Follower and Subscriber Settings window:

Here, you can change the settings for followers and subscribers by selecting their icon (please note that only certain Twitch partners have access to Subscriptions):

Set their visibility options:

Choose the styles and opacities:

Configure the sounds used:

And set the welcome message for new followers and subscribers: