The first step to drawing is to have a photo or image that you would like to draw.
Here is an example:
Found at: https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=0ahUKEwj0nOPVotfTAhXHRhQKHYu9DaAQjhwIBQ&url=http%3A%2F%2Fwww.graphicsfuel.com%2F2013%2F12%2Fcoffee-cups-psd%2F&psig=AFQjCNEByINifjzZs1vTBkymVwMz2rhO6w&ust=1494022619698688
Once you have the picture or image you need to choose what proportion or part of the image you are going to draw. Look at the example below. You may only want to replicate a portion of the object or photo.
On your paper, map out the perimeter of your drawing proportional to your photo/object. Make sure the vertical and horizontal measurements of your drawing fit within the borders of your paper.
Find parts of the object/photo that look the same size. For example, the length of the woman from head to toe is equal to 7 1/2 of the length of the woman’s head in the picture below.
Found at: http://www.learn-to-draw.com/figure-drawing/05-draw-female-proportions.shtml
Here we have different proportions of the male figure, ranging from 7 1/2 to 9 head units. Keep in mind the number and size of the head units will determine the proportion of the figure to the rest of the drawing.
Here is another example: the height of the unicorn's horn on both side are the same length. If you make the the height of the unicorn's horn a unit to measure other parts of the unicorn, you can see 2 of the horn units make up the length of the snout from the ear to the nose and 1 horn unit of the bottom of the neck. The back of the unicorn's neck would probably be equal to 1 and a 1/2 of the horn unit.
Found at: https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjpwM3q_NjTAhVFTSYKHTOrDHoQjhwIBQ&url=https%3A%2F%2Fwww.pinterest.com%2Fexplore%2Fgeometric-drawing%2F&psig=AFQjCNF8IvGzNb3nX-ZNgUsUIsZp1JKNwQ&ust=1494081262648955
Any object in your photo can be used as a unit of measurement to find the proportion of your drawing. Each drawing is unique; therefore, the dimensions are going to very for each drawing.
Once you find the portions of your object or photo that you are drawing, then you start making points and measurements on your paper that are proportional within the broader or perimeter of your drawing.
For example, with the unicorn, I would decide what length I want the unicorns horn to be and then use that horn unit to measure the rest of the unicorn's outline and shape to give me the right proportions of my drawing. Continue this measurement system until the drawing is complete.
HAVE FUN DRAWING