What's the best advice you can give to someone learning to create the icons?

You guys have more experience than anybody. So?

Hi! To begin with, you can read a book called The Icon handbook, here it is: http://iconhandbook.co.uk That book is not free but you can find some free options as well! This one, for example: http://iconutopia.com/free-icon-design-guide/
Then you can also check out our article here https://icons8.com/articles/design-icon-set-simple-checklist-graphic-designers/ and to watch this video https://icons8.com/articles/make-pixel-perfect-icons/ (the video is placed at the end of the article).
Also we would recommend you to read Google system guidelines for material icons, for example. It can give you some useful tips as well!
Good luck! :wink: