How do you generate all sizes app icons for ios and android mobile apps?

First, you should have a great design for your icon and file should be in PNG format with size 1024 x 1024 pixels. 

Second, use a free online tool like https://makeappicon.com/ or http://appiconmaker.co/ to size down your icon for all different sizes for the Apple App Store or Google Play.

Finally, upload them when you submit the app.