To enable them, first make sure you update the Snapchat app and have Bitmoji installed. Next, go into the Snapchat settings (the gear icon on your profile page). Select the “Bitmoji” option:
You can send the Bitmojis in chats, or add them on as stickers onto your snaps. To add them as a sticker, take a picture or video, then hit the turning page icon (next to the “T”). This lets you add the big emojis. Go to the face icon to find your Bitmojis.