Step 1: open that Snapchat-App on your smartphone.
Step 2: Tap the Chat-Symbol, which is located at the bottom of the screen next to the camera button.
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Step 3: Next, open the conversation where you want to use the Bitmoji reactions.
Step 4: Then, long press on the message you want to reply to.
Step 5: Just tap the appropriate one Bitmoji for your reply, and it will to appear direct under that Embassy in conversation.