When learning to draw observing your subject correctly can be a challenge in itself
. This is because you can get in a habit of making "mental notes" about objects. Usually there is too much information for our eyes to take in and process so you mind registers only the essentials. If you choose a subject that is familiar it does not become easier as your mind becomes lazy. There is a few approaches that can fool your mind out of drawing what it remembers so that you draw what you really see.
1.Measuring Relative Proportions
The "thumb and pencil" method of measuring the relative lengths of the subject. You can then use the established guidelines you have drawn to sketch in the features or objects in the correct space.
2.Use a Grid
Prepare a grid of one inch squares on white cardboard to place behind the subject or make the grid up on glass to place in front of the subject. You will be able to use this grid as a point of reference. This will help you make more accurate judgements. This grid can also be used on your canvas, paper, etc.
3.Checking the Mirror Image
It is surprising how different your drawing will look when viewed with a mirror. This is a good way to check your accuracy. Everything will look unfamiliar and errors will stand out.
4.Drawing Upside-Down
This is another way of fooling your mind while it is upside down it looks much less familiar. This can be an effective warm-up exercise. Take a picture or photo turn it upside down, Copy it carefully the drawing you are producing would also be done upside-down.
Hopefully what you have read will help but I have also included a couple of links that will go into more detail. They are definitely worth a look. I also included a link to my blog where i show my own progress.