Hi Nichola: Start a new project. Go into your Project Settings, click on Canvas Dimensions, click on custom, and change the settings to 1080 x 1080, then apply. That would be your settings for Instagram.
Then save the project as Instagram. The next time you want to do an Instagram video, open the project Instagram, save it with your new video title, and edit away! Render out the video in MP4 format and load that to Instagram.