The very first thing I do with a new art student is to help them learn how to draw proportions. If you want to draw what you see, this is a skill you must master.
I have seen people struggle for hours trying to fix a drawing because they have never been taught how to measure proportions. A lot of time and money is wasted in art classes where the teacher tells you to “just eyeball it.”
Once you learn how to draw proportions free hand, then you can use drawing aids such as grids, projectors, or proportional dividers to improve the results.
It might seem a bit inartistic to spend so much time measuring ratios, especially if you are accustomed to jumping right into a drawing. That's normal; I had the same struggle when I took drawing lessons. Anybody can do this, however...including you!
In the demonstration below, I'm using a very simple set up. Once you grasp the concept and practice it, you can learn how to draw more complex subjects in any genre.
Block-in stage. The first thing I do is look for some feature on my subject that I can use to reference my measurements. In this case, I choose the height of the apple.
On my drawing paper, I mark where I want to place the top and the bottom of the apple. Then I use my handy-dandy drawing pencil and measure the height of the apple. To make sure I get the perspective correct, I lock my arm, close one eye, and stand at the same spot every time I measure.
Great! Now I have a unit measure for this drawing. Next, holding the pencil with my thumb and index finger and turning it horizontally, I line up the pencil tip with the right edge of the apple and compare the distance to the left edge relative to the height. It is about the same, (roughly a 1:1 ratio).
So I go back to the drawing paper and make a point on either side of the vertical line (with the distance from side to side about the same as from top to bottom).
The rest is easy. I use the same unit of measure (the apple's height) to get the height and width of the blue bottle.
Then I measure the rough proportions of the bottle (I emphasize 'roughly' since it does not need to be too precise) by measuring how far the neck extends from the top compared to the full length of the bottle. It looks to be about 1/3 of the length, so I mark it accordingly on the paper.
Next I break the narrow part of the bottle down to two parts: the distance between the top of the cork and the lip of the bottle and the distance from the lip to the place where the neck widens.
Now it is time to connect all the marks with small straight lines. Use bold straight lines if you can. It is harder to draw curves accurately at this stage, so if you try, you are more likely to make mistakes.
To measure the slant of an edge on your object, just tilt your pencil to line up with the edge and then bring that to your paper and record it. You do not need to worry about whether it is 30 degrees, 45 degrees, or any precise measurement, just mark on your paper what you observe with your pencil.
This little art lesson is no substitute for a real art instructor teaching you how to draw proportions, but, however you do it, it is important to learn to draw correct proportions right from the start.
To sum up the process:
Look for an obvious unit measure—a length or width of something important in the subject (in figure drawing, artists often use the length of the subject's head)
Begin by measuring the ratio of big masses and then break them down to smaller fragments.
Keep the perspective constant by holding your arm straight, closing one eye as you measure, and standing at the same “station” point. Be careful: if you shift the distance or perspective, the ratios will be off and you will have to measure everything again.
It's vitally important that you note all the important “landmarks” that indicate the width, length, and end points of edges before you start to delineate the form.
Use ratios of 1:1, 1:2, 1:3, 1:4, etc. Don't drive yourself crazy trying to measure extremely precise ratios! For that sort of thing, you can use mechanical aids such as a proportional divider, a grid, or a projector.
Measure twice; mark once just like the carpenters' motto. Often, you don't measure it correctly the first time, so it you get the feeling that some proportions are off (and it happens often), just measure again.