"Writing into stone" some tools and links that have help me learn and improve my designing skills. This can actually come in handy for any other product designer out there, so let me know if it helps!
Intro to the 8-Point Grid System
Your new design religion. Medium article by Elliot Dahl.
Making a case for letter case
Medium article by John Saito.
7 practical tips for cheating at Design
Medium article by Adam Wathan & Steve Schoger.
How to develop an eye for Design
Medium article by Kathleen Warner.
Design techniques by Area 17
Amazing descriptive guide on their practices of design.