Drawing geometric shapes in perspective is much
easier than more complex objects, so we'll start with a
plastic bottle cap. When you look at the cap straight
on; when it's right at eye level and therefore at the
horizon line, it looks like a rectangle because you can
only see two dimensions: Height and Width. If you look
at it from the top, that's when you can see the third
dimension: Depth.

Let's draw this bottle cap 


here's how 
So
lets draw the bottle cap three times  straight on, from
the top, and from the bottom. On a new piece of paper,
take a ruler and draw a straight horizontal line in the
middle of the paper. This will be the horizon line. Take
your best guess as to where the center of the line is
and draw a dot there. That's your vanishing point.
Now
Freehand draw as perfect a rectangle as you can with the
center of the rectangle being the vanishing point. Make
the rectangle as tall and as wide as the bottle cap
looks to you. There! you just drew the straight on
bottle cap! Now we'll draw the top and bottom views.
Draw two parallel lines and extend the two sides
of your cap drawing well above and well below the
rectangle. This is done so that the top and bottom
view caps are the same size as your first cap
drawing. Now guess and put a horizontal line above
and below the cap drawing. This is where the middle
of each cap drawing will be.
Draw
an oval on both top and bottom horizontal lines so
that the line bisects the oval.
Now
on both the top and bottom drawings, draw the same
sized oval below the oval so that the top of the new
oval just touches the bottom of the first oval.
On
the top oval erase the bottom of the original oval,
and on the bottom one, erase the top of the original
oval.
Erase
the two horizontal lines on both the top and bottom
caps, and add a smaller oval inside the top view
cap, and there you have it! you've drawn the three
views of a bottle cap! 