Directions
If you open your AE file and if you are ever missing files, select the file missing in your top right file bin and press ctrl + cmd + h and select the file (video). Make sure all of your files are saved on your Macbook and it helps if they are backed up on Google Drive so you don't have to start over in case your Macbook breaks.
Remember that cmd + k will open up your settings if you need to change the AE composition size, background color, time duration, etc.
Do not do
When finished, render as H.264.
but if your video file size is too big (e.g. over 200mb) then open your .mov video in Quicktime & export it in 720).