Please watch the video above first. You can freely download the accompanying images below for reference and practice.
Start with sketching. Take your time and practice. Don't expect good results right away. Give it time.
A chart displaying difficulty of subject matters. In general, more organic and less familiar subjects are easier. More structured (heavy perspective) and familiar subjects (human face) are more difficult.
Drawing tree branches and roots are relaxing and easy. You can use these references and give it a try (reference images were obtained from Google Image Search and do not belong to me).
Another page of root, branch and tree references.
Instead of starting with details, capture the major forms and silhouette first. In addition, estimate the horizon line so you are aware of the eye level (indicated by the red line).
Use soft lines to rough out the major forms. Try to keep your wrist and arms relaxed and don't hold the Wacom pen too hard. Let the lines flow naturally. Once the major forms are captured, apply line-weight.
Adding contour lines help define form. Finally, apply a grey marker wash to "pop" the image against the white paper.
Another example, this time using reference.
Take your time and finish each sketch nicely. Don't rush. Everyone improve at different rates. The key in the beginning are good results and not speed.
Additional tree branches and trunks examples - using the references posted earlier.
Examples of random sketches from my personal work. Draw and sketch what you enjoy. You don't need to follow trends.
Having fun sketching what I want.
Organic subject matters.
An example showing the importance of sketching. These are my functional breakdown drawings of General Grievous from the Star Wars movies.
My sketching brushes. Everyone has their own preferences when it comes to Photoshop brush. Find what works best for you.