Most beginners seem to have difficulty seeing things in perspective. At
least it seems to be the area where most drawings go terribly wrong. So
let's deal with it.
Good books teach perspective mathematically and geometrically. One of my
sons, who is an architect, gave me a book on the subject which enables one
to construct complex structures in perfect perspective. I cannot
understand most of it.
There are simpler books for artists and I recommend finding one if you
truly wish to pursue the subject seriously. Most of them start with the
important "two point perspective," which shows how receding lines
intersect at the horizon. I understand that, but I seem to have little
success teaching it to other people. So I stumbled upon an easier method
for the casual sketcher which is very practical to use.
Cut yourself a nice little mat frame out of cardboard with seven inch by
ten inch outside dimensions which can be conveniently carried inside your
watercolor block. Mark it according to the illustration below. Also cut
yourself a simple cardboard strip to go with it. If you wish, mark it with
a graduated scale, which can help you in determining the relative
proportion of objects you are
Use the frame to pick and
compose your subject. But also use it as a perspective device. Hold the
frame perfectly horizontal and view your subject through it. Then place
your cardboard strip over it to exactly match the angle of the roofline or
other item in perspective. Hold the strip firmly to the frame and move it
down to your paper---and draw a line along the edge of the cardboard
strip. This will exactly (if you have done it right) duplicate the
perspective line of the object you are sketching. It sounds complex,
perhaps, but it is terribly easy once you get the hang of it. Study the
illustration below. With this simple device---and a little care---you
can't go wrong in drawing in perspective.