The below resources provide a range of different approaches and fidelity levels to build skills and confidence in sketching. 
Lectures and tutorials that highlight the thinking and techniques used to create drawings.
Sam Gwilt’s channel with tutorials, walk-throughs, and how-tos and his thoughts about design.  
Lots of great sketching examples
The Advanced Design Sketching channel has more of Reid’s videos and other designer’s tips like Spencer Nugent and Hector Silva. All the videos are full of insight into design thinking and sketching.