Learning mechanical drawing requires learning many basic drawing techniques such as perspective and proportion. Learn to Draw offers some basic lessons on how to use space, how to measure distances, how to capture proportion and how to work with perspective. An example exercise is to measure proportion by holding your pencil out at arm's length and look at the object you're drawing. Use the pencil to determine how large the shape should be on your paper. Although this is a good introductory program into general drawing techniques, mechanical drawing requires more precision. To practice this precision, you may wish to attempt some of the detailed perspective exercises offered by Kevin Hulsey. These lessons are specifically oriented toward learning technical drawing and thus discuss practice and techniques more fully.
Self-Directed Study Online
Another option for the aspiring mechanical artist is to learn how to replicate designs that are featured online. Look for basic mechanical diagrams that will help you understand how the patterns and appearances were created. The University of South Florida, for example, offers 180 mechanical images that you can work to duplicate. These exercises provide you with a finished pattern, such as a series of nested circles, and hints regarding how to re-create the pattern at home. An important tip to keep in mind when using tools such as triangles or curves is to keep your pencil in the same vertical orientation to the tool as you trace its edge. As you work to get elements of perspective and shading more refined, you can either make multiple copies of the shapes you've made on a machine copier or you can print them from the Internet.
Invest in Books
If you feel you have progressed as far as you can trying to work from these types of exercises, you can invest in a self-help book. Several books on the market are designed to help you achieve stronger mechanical drawing skills. Be sure to pick out one such as "Mechanical Drawing Self-Taught" by Joshua Rose. These kinds of books focus exclusively on mechanical drawing, discuss the tools you will need to succeed and provide detailed instructions on how to achieve the effects discussed. One of the advantages of older books like this is that they don't presume you have a computer handy with CAD software installed. Instead, the book attempts to teach you to create these designs by relying on nothing but the mechanical tools listed in the introduction.