The best way to get better at sketchnoting is practice - practice - practice! Practice your listening skills, your content structuring and drawing.
There are a lot of good TED talks that can help you train your sketchnoting. Just browse through them and choose one you would like to visualise!
Then go ahead and take your sketchnote and favorite pen and simply start sketchnoting. Use the pause button, rewind if you need to hear a certain word again and remember to take your time. Focus on the key messages and not your drawings. You can always go back and go over your icons or letters and hide a mistake with colours or a shadow etc.
Not feeling creative enough for a new TED talk yet? Well then: How would have your Sketchnote of Steve Job's talk looked like?