Pick a shape: 9:16 for Reels, Shorts and TikTok, 1:1 square, 4:5 for the Instagram feed, 16:9 for YouTube, or 4:3.
Then decide what fills the gaps. “Blurred background” is the popular choice: the whole clip sits in the middle over a blurred copy of itself. “Black bars” is the classic. “Crop the edges” fills the screen and trims the sides; choose whether to keep the left, centre or right.