Sketching and drawing with correct proportions and doing so without reaching the end of paper can be achieved easily with one simple technique. Before getting into the technique, let's answer 2 questions.

1. How many times has your drawing gone out of the paper?

2. How many times does it happen that you get the overall shape of the object all wrong?

For many of us, the answer to both these questions might be "many times"."Envelope technique" solves both these problems easily. 

Envelope Technique 

Imagine that you are wrapping the subject of your drawing in a plastic wrap or imagine that you have to draw the outer shape of your subject using only 6-8 straight lines.

  When you draw the outermost shape of the subject, what you get is an envelope. Depending on the size of the paper you are using, you can draw these envelope lines longer or shorter. 

This establishes the extreme ends of the drawing on your paper. It helps you see the big shapes and small shapes, it helps you to see the relative proportions of shapes and also establishes the dominant angles in your subject.

 Once this is established, there is no worrying about reaching the end of the paper, incorrect proportions or fear of rework. After this point, it is more of the same. 

Go on finding progressively smaller and smaller envelopes till you get all the details of the subject. Draw only straight lines, even if you see curves in the subject.

Remember that no line is final unless you decide to call it final. You are the artist and you have the authority to correct the lines that you drew earlier. Once you feel that you have drawn enough details, make the correct lines darker to finish your work. This is just like a sculptor who removes unwanted parts of stone to reveal the sculpture inside.  

Drawing is more about observation skill than anything else. The more you learn to observe your subject, the better your drawings will become.