Most artists have probably never considered using a 3 inch house painting brush for their oil paintings, but doing so can get fantastic results with painting skies.
A soft bristle brush enables wonderful blending of the sky colours. Just start by thinning your white paint with a blend of 1/4 artists turps or gum turps with 3/4 linseed oil. Paint a very thin layer of white paint over the sky area and also over any other areas of the painting which will be light in colour.
Next select the colours you wish to use in the sky. Yellow ochre for the bottom, red violet for the middle and either blue or blue violet for the top will give great results.
Again thin the paint before applying to the white on the canvas. Remember that the ground or river colours are a reflection of the sky colours so as you apply each colour to the sky, starting from the lightest colour and going to the darkest, also apply this colour to the ground or river areas.
With the sky it is best to use criss cross strokes, and where one colour moves into the next use very light strokes with the brush to get a good blend between the colours.
For the ground or river use horizontal motion.
Clouds can be put into the sky once complete using a fan brush and then use a soft bristled fan brush to stroke them in a sideways motion to blend into the background colour of the sky.
Keep in mind also that shadows on the ground also pick up colour from the sky, so mix the darker colours such as the red violet and blue violet or blue with some burnt umber to use for shadows on the ground or for the opposite side of the trees to the chosen light source.
The light colour of the sky, being yellow ochre can be used as the highlight colour for the foliage on the trees.
The dark areas of the painting such as thick treed areas should be painted with a background of either burnt umber or a mix of burnt umber and dark green. Apply this coat as thinly as possible to prevent a muddly look when the contrasting colours of the foliage are applied.
Using a 1 inch or 2 inch brush to highlight the foliage areas can give much better results than using the normal smaller artists brushes.
Give these techniques a try and see how you go.