Sloppy A
Lets first try drawing something really easy, a sloppy "A".
This tutorial is dont by just turning on the line grid and and
drawing three lines. If the
viewpoint is "identity" position, zoom out the viewpoint to show more grid squares.
Notice that the grid is regenerated to fill the viewport. If nothing has been changed,
the grid spacing is set to x = 1, y = 1. The units are whatever you imagine them
to be; the example figure is either three miles tall or three centimeters tall
depending on how you conceive of the model space units.
Lets draw the first line. To draw a line, use
the line function 
under "Polygon Primitives". When you invoke this function
a dialog box appears asking for input. The input to this function is two points in
space. The first one is highlighted in the dialog because this data entry device is
listening to the mouse and the keyboard. Anything you type now will appear in
this box. When you move the mouse the mouse position is used to calculate a point.
Notice that as you move the mouse around a red cross hair appears over it, and
further notice that the lines of the cross hair a parallel to the lines of the
grid. If you rotate the viewpoint to a new orientation now (there is no need to
cancel the line function first) by dragging the right mouse button, and move
the mouse again you will notice the red cross hair is still parallel to the
blue grid lines. You would be doing your self a favor to turn on the user axis
tripple so you can see where the origin of coordinate system is. The point in
space is calculated by comparing the position of the mouse and the user axis triple.
As you move the mouse around, the point read out is updating twice a second to indicate
the point you propose to enter by pressing the mouse button. (The point read out
is in the highlighted data entry widget.)
The notice that the z value of the point is always zero. This
is because you are picking points using a picking plane that is the x-y plane of
the current coordinate system. Go ahead and pick the first end point of the first line.
When this is done, the focus changes to the second entry widget of the dialog,
inviting you to pick the second end point of the line. Do this, and the line is done.
Now lets do the second and third line. Call up the line function again but this
time check the "multiple" check box on the bottom of the dialogs. All primitive
creation function in CADZilla come with this check box. If it is enabled, the
function will be automatically re-invoked upon completion. Draw the second line,
and the dialog is still up ready to draw the third. Draw the third line.
You will need to hit cancel to dismiss the dialog since the multiple check
is on.