One of the easiest and most foolproof ways to check for mistakes is to look at your drawing in a mirror. Start with an underpainting of a harmonious color which will provide a foundation and provide the atmosphere for you painting. For a landscape, an underpainting of ocher, sienna or umber will set the tonal values. Limit your palette to two or three colors at the early stages of your painting. More intense colors can be added in the later stages.
Many painters work from the top left down to the canvas bottom.
Others work on all sections of the canvas simultaneously. These methods work best if you can envision the end painting or have a detailed drawing on your paintings surface. If only specific elements can be envisioned, start with what you know. This is not uncommon in non-representational work which is more spontaneous. Placing the prominent elements will help you decide how to unify the painting.