OK, I'll try to get this subject back on line.
Cad programs fall into two main flavors 2D and 3D. Examples of the 2D ones are early Autocad, Turbocad, Autosketch, Draffix, Easycad, Fastcad and plenty more like this. Examples of 3D are later Autocad, Turbocad, Cadkey and others.
Next up the list are the modeling programs often called 3D but there is a difference. Classic examples of this are Solidworks, Solidedge, Alibre etc. The reason these are different is that instead of working with lines to produce a drawing you are working with shapes to produce a model. Any drawings needed are taken from this model. These except Alibre are all high end programs as regards cost.
Going back to basics here and addressing 2D only as a starting point.
Many books will explain how to start off drawing but they all assume no one has ever drawn before and start off teaching the relative method of drawing. That's where you start off from a given point, usually 0,0 and draw using a series of relative points something like X0,Y2 will draw a vertical line 2 unit tall, X2,Y2 will then draw a horizontal line 2 units to the right etc
What no book I have read takes into account is that a lot of us have had years of experience on drawing boards and we are used to CONSTRUCTION lines, shades of 6H rubbings etc.
The obvious conversion for such people as this is to set a layer up called construction and use that layer in the same way. Swapping later on to the main or outline layer one can draw in the true shape and either delete or preferably just hide the construction layer.
A classic method to suit these people is to set this layer up, draw one horizontal line and one vertical line starting at the ref point of your part, called 0,0. From these two lines by selection a common command in CAD systems, Offset, you can quickly and accurately off set the two lines to produce the main parts of your drawing.
This method doesn't always suit everyone and it pays to take a little while to try the two methods to see which suits the best.
-- Regards,
John Stevenson Nottingham, England.