The rules per Asher B.Durand

1. Draw, draw, draw.
2. Don't be after money.
3. Direct imitation of foreground objects in bright light with good shadows for starters.
4. Avoid colour and broad scenes at first.
5. Eschew the picturesque in favor of rawer nature.

Guide to foreground objects: rocks, tree trunks, and after these earth banks, coarser kinds of grass with mingling roots and plants, larger leaves. Avoid running water, masses of foliage.

Then he gives you the rules of landscape. It's hard to describe how things are seen when painting. That was the fun of reading this. ( )