You will find this is a great opportunity to take your painting to the next level. Even if you prefer studio painting, give this a chance, it will delightfully surprise, challenge, and improve your observation skills and color choices.

Before you begin the following items are helpful in creating wonderful art. This is not a competition, art is individual, I can help you with the mechanics of painting. But the rest should be a reflection of your own inner voice

Relax, Have Fun, Remember to Breathe!   Keep it Simple

Have in mind:

• Concept or Theme—
  An overall view of what the finished painting will look like.
  
  
• Value- Color - Design—
  Analyze and understand Value structure to understand color, then employ both into design in a way that enhances your concept or theme.
  
  
• Drawing - Edges - Perspective—
  The painterly tools the refine and control the Value, Color and Design of the Concept or theme.
  
  
• Support Cast of Pictorial Tools—
  Balance, communication, contrast, discord, dominance, emotion, exaggeration, center of interest or focal point, gradation, harmony, overlap, repetition, reserve, symbols, texture, and variety.

All of these above painting terms, elements and tools contribute to a "sense of place" the most important quality in landscape painting.

Don't forget to squint!  Always do a quick sketch first!   Begin with a quick sketch of your chosen subject, sometimes two or more sketches, to get the correction composition, shade and light value areas. Using a view finder will help you isolate the overwhelming landscape and simplify the painting subject. Look for contrasting blocks of color values and interesting shapes. Keep your sketches simple and do them quickly.

Canvas Prep:
Especially for plein air painting, the white surface of canvas can be harsh, and will influence your color choices. Tone down the white with a turpentine wash, Burnt Sienna, and then wipe it with a towel.

Canvas drawing:  Use your brush, and a thinned down paint, in a medium brown tone. Again, only draw in the simple shapes of your subject.

Palette Preparation:  Premix your colors for the painting before beginning. Start with the darkest color value, then move to a lighter value of the same color, Then the lightest value, and any other colors, medium values you see before you.

Identifying Color Values:  This is where a lot of us struggle. I've found it useful to create a few color guides to help with this; a black and white color scale is also helpful with this.

Painting:  Begin Painting with the darkest Color values in your setting.  Block them in, and then move to the next value, gradually working toward the lights.  Step away from your painting when you are almost done or if you reach a frustration point. It will help you gain perspective.

Finishing: When you are almost finished, add in any extra accents, details, then quit.

Varnishing:  When your painting is dry, varnishing will protect in and even out the highs and lows that happen with use of thinning your paints, using mediums or even colors used. Be sure to use the newer synthetic resin varnishes that are non-yellowing and won't crack.

Name brands: Kamar, Golden, Winsor&Newton, you can use Gloss, Satin, and Mat finishes.
Personally, I prefer the spray on, it can be used before the painting is dry to the touch but not yet completely cured.