The below resources provide a range of different approaches and fidelity levels to build skills and confidence in sketching. 

Learn to Draw Series
Check out this four part Youtube series by the Concepts App. This series gives a great introduction to sketching and shows how you can develop the skill to communicate your ideas with yourself and others creatively.

Scott Robertson Channel
Lectures and tutorials that highlight the thinking and techniques used to create drawings. 

Sam Does Design Channel
Sam Gwilt’s channel with tutorials, walk-throughs, and how-tos and his thoughts about design.   

Hakan Gursu Sketching Channel
Lots of great sketching examples 

Advanced Design Sketching
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.