Perspective is what gives the painting a feeling of depth. In order to do this you need to find the horizon and the vanishing point, which is usually at your eye level. All lines which look parallel actual are heading at various angles towards this vanishing point. I have found that sometimes the vanishing point is not actually within the area you are drawing, which makes it harder….
There is some very good information on here:
which includes this Van Gogh with perspective lines drawn on.
I have downloaded the pdf from this website for future use, which is excellent.