I believe the best way to get better at sketchnoting is to put your pen to paper and draw. Ultimately, the way to get the hang of what style of sketchnote works best for you and your subject matter is to try things out. To give you a feel for how my process works, watch as I capture sketchnotes in real-time from a talk presented by musician Jon Mueller.
As you watch, note the order in which I work, the details I choose to capture and the details I choose not to capture. Remember that your focus in sketchnoting should be to capture the big ideas, analyze them and place them on the page visually.