Some recommendations for beginner plein-air painters like myself.
Plein Air Supplies
Everything must be small. Collapsible cup, 4 brushes at the most. A small palette.
Zucca bags are convenient. A three-leg stool, a small foldable plastic table.
The Pro Plein-air bag is widely used. Expensive but compact.
Plein-Air Painting Approaches
The serious plein-air painter will have a pro kit; frequently their medium of choice is oils. However, a new current of urban sketchers have popularized watercolor sketchbook practice with outstanding results.
Sketching with watercolors influences the way you will paint later. Sketching loosens your mind and hand up, it gives you confidence, you will draw better. Also, the care-free attitude helps. It is just a sketch!!
Scout the location of your choice and figure out how the shadows of trees and objects around will fall on your paper. When under a tree, branches and leaves will be cast shadows on your canvas or paper.
Zero in on a small subject and modify all things that surround it to serve the focus of interest you have chosen. Let's be less ambitious and more focused.
Once you have in your mind's eye the image you want to paint, disregard the actual scene and start working with what you have on your paper. This is your sketch now. This is your vision.