To customize your videos select the video you want to customize
Go to the right side panel for the video customization options
You can make any customizations as you need.
You can then customize what you want :
Position and size
Rotation
Proportions (which permits you to choose whether the video is contained in the story or covers it)
Border (its color, width and rounding)
Opacity
Once you select your video, there's also a control panel which will appear at the bottom in the middle of the screen. This control panel lets you edit the video. You can :
Choose which part of the video you want to use in your story by selecting the starting and the ending moment ;
Decide if the video will loop (meaning restart from the beginning once it's finished) ;
Decide if the video will autoplay (meaning start without the need for the user to click on a play button).