Don’t paint small landscapes. 90 x 120 is best.
Square up the canvas, 4 across and 4 down.
Then square up a piece of plastic or glass of the same relative dimensions but small of course so you can fix it to your easel. (the Old Masters made a small wood frame and stretched string across.)
Look through the frame and draw what you see in the squared frame square by square onto the canvas in the same squares. The frame must be the same distance from your eye each time.
Put a knot in a piece of string and put it in your mouth fastening the other end to your easel. Later you won’t need to do any of this because the squares exist in your minds eye.
It is far better to paint a landscape on site or at least from a sketch or a photograph you have taken yourself. A landscape talks to you when you are deeply part of it.
A kind of soft moaning sound and the sky makes a lighter sound taking you far far away. That doesn’t happen if you haven’t actually been there and so a remoteness pervades your painting.
Remember a landscape is a living thing and you should feel it.
If you don’t then perhaps you should be a tax collector or a dentist not an artist.
You can of course paint portraits by squaring up, like German artist Durer used to do.