If you want to use the Bitmojis, you have to download the app from the App Store first and then create an account
How do I create an account and my Bitmoji?
The stickers can be used for all your messenger services like WhatsApp. The Bitmojis are designed to function as an own keyboard like the standard emoticon keyboard. In order to use the Bitmoji keyboard on your iPhone, you have to activate it just as any other downloaded keyboard.
The most interesting part is allowing the access for Snapchat because you cannot only use the stickers. You are also able to use individual filters with your avatar, for example for every weekday. If you have friends who use Bitmoji as well, you can send each other stickers that combine both avatars. So you can send stickers displaying an activity together. Since the Snapchat update (10.4) you can see friends in the Snapchat widget of the Today View.
So Bitmojis are a nice change to Apple‘s given original emoticons and give your messages a personal touch. Especially Snapchat users will love this app because Bitmoji offers more options to design snaps.