From the Messenger Home screen (where you see a list of your conversations) tap the Camera icon toward the top of the page—this particular Camera icon says Add to Your Day underneath it. Tapping this icon opens the camera interface. You can scroll back and forth through various photo effects—masks, frames, and color effects alike—when you find one you like, tap on it, then tap on the center circle button to take your photo. You can hold the center button to record a video.
After you take your photo or video, you can choose to add stickers, text, or doodles to it using the icons at the top of the screen. You can choose to abandon that photo by tapping the back arrow or X in the upper-left corner. Save it to your phone’s camera roll by tapping the Save icon in the bottom left. But assuming you took a photo because you want to share it, tap the arrow button in the bottom-right to decide who will see it.
By default, Facebook assumes you want to both add it to your day and save it on your camera roll. You can also choose to send that photo or video as a message to specific friends by tapping their names from the displayed list. Once you’re confident with who you’re sharing with, tap the send button in the bottom right.
Sharing things using the My Day feature is a way to share things from your day without inserting it into people’s News Feeds. People using Messenger must click on your day to see the photos you share. In other words, you won’t be annoying people unless they are choosing to be annoyed by you. People also like the way that sharing something that disappears means they don’t have to share something perfect.