a sketch is an impression, a feeling, not a technical finished drawing
freehand Impressionistic Sketching
A sense of freedom, a sense of feeling, a sense of fun - not bounded by constraints of what you may have been taught or thought.
Sketch what you see
Sketch what you see - but in the way you see it. Sketch what's there, but not everything in detail - sketch the way it makes you feel. And don't plan too much - if you plan in detail in pencil to remove later you may erase the feelings that should show how you felt.
want to know more about freehand sketching - check out 'sketching' posts on the blog.
one and two point perspective
Create a sense of being there. If you're drawing what you see it will be in perspective,
permission to draw from photos
You can't always run around the world looking for the best things to sketch. You can sketch anything you see - but that still becomes finite and not always that exciting. Draw from photos - it is allowed - you can give them a significant context through Worldbuilding if you want.