I put this tutorial together after my first perspective class because I thought that this information makes perspective drawing much easier to understand. In my class, emphasis was put on making perspective your slave, not the other way around. When you're putting a composition together in perspective, it's easy to begin adjusting your characters or objects so it "fits" within your perspective limits. In this tutorial, I'll try to give you an idea on how to make perspective fit to your composition. For instance, say you sketch out a thumbnail drawing of your composition and you like it. Start out with the figures, find out where the eye level and reference points (or vanishing points) are and you will have created the foundation for an accurate drawing.
I'll show you a bunch of simple concepts which you'll be able to apply for more advanced drawings.
The first thing that you need to get an understanding of is what the eye level (or horizon line) and reference points (or vanishing points) are.
Eye Level
I'll start with eye level. It's pretty self explanatory. The eye level is a horizontal line that represents the horizon. The eye level is basically the height of the camera. For example, say I'm 6 feet tall and I take a picture standing up. The eye level of that image that I take will be about 6 feet from the ground. Now, if I squat down to about half my size (3 feet) and take a picture, that image's eye level line will be from three feet off the ground. It's important to determine where the eye level line is when you're drawing a picture in perspective because that is where the reference points will be located.
Reference Point
The reference point (or vanishing point) is a point which lies on the eye level line. All objects in every image recede to these reference points. There can be one, two or three reference. points. With the eye level and reference points found, you can place any object you want accurately in perspective.
The eye level runs right through the eyes of the subject in this example. This means that the subject has the exact same eye level as the camera does. If the eye level was higher, that would mean that the camera is higher up. If the eye level was lower, the camer is lower.
In the example on the right, the reference point was found on the eye level and dashed lines were drawn from the ref point to the figure. Now that you see the receding lines, it is easy to visalize more figures in the background (bottome). This is one of the basic concepts you need to understand.
Adding a new figure in perspective:
We now know how to find figures using one reference point but obviously, people are not always standing in straight lines. In this next step, I'll show you how to place more figures using a second reference point.
First, decide where you want your next figure to be located. Look at your compositional sketches and make sure you follow it. Don't become a slave to perspective by forcing your objects to fit. I made a mark on the page that indicates where the feet of my next figure will be located. Next, I located a second vanishing point. You can put this point anywhere you want just keep in mind that the closer the two points are, the more skewed your perspective will be. I kept them kinda close for the sake of this tutorial.
Next, I drew two lines that recede back to the vanishing points so I can see exactly where this new figure will be in perspective. Since we have the bottom of the new figure located, all we need is to find the top of the head. To do this, we have to use one of the figures we already have as a reference. Do you see how our two figures fall on the same receding lline? Well, that is the same conecept we'll have to use to find our third figure.
Observe the new lines that were added. Basically, I need to find where the first figure would be located relative to the third figure's receding lines. After that, I can use the top point of the first figure and move it down to find the top point of the third figure. Get it? Here's another graphic to clear 
it up.
Finally, we've found our third figure. Once you get used to this concept, it's really easy. It's so easy that you'll probably be able to free hand it eventually. Here are our three figures. Once you have your figures established in perspective, go ahead and use the same vanishing points for the backgroung (buildings, trees, etc.). It all works the same way for the most part.
Common Eye Level
When you are trying to draw many people in one drawing, a good trick is to find a common eye level if there is an obvious one. For example, if the eye level runs accross everyone's shoulders, then you don't really need to know anything else. Just draw in the different sizes making sure the shoulders line up. As you can see, the perspective looks right. The same thing goes with say, feet, or hips.
Working with Varied Heights
We have to take into consideration that people vary in height. So, once you understand this next concept, you'll know how to deal with that problem. This example includes a father who is about 6 feet tall and his daughter who is half his size (3 feet). I think it's best to generalize/simplify the sizes like that.
First, draw in the daughter and then the father in the background. We'll need to find our eye level and reference point so we can create an accurate background once we're done with the figures. Since the little girl is half of her father's size, we'll have to add 3 feet on top of her head. This new point represents 6 feet. Since the father is 6 feet tall, all we have to do is draw a line from that point past the fathers top point and another point passing both of the bottom points. The intersection of these two lines marks the reference point. Draw a horizontal line through the reference point and you have your eye level. Easy! The same basic concept can be used for the reverse. If the girl was in the backgroun, just add three feet on top of her head or you can subtract half of the father's size to find the ref point.
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