A camera lucida reflects a subject so that it appears to be on your piece of paper, enabling you to simply trace it. The following is based on using a camera lucida made by The Camera Lucida Company, but they all work similarly.
Setting Up a Camera Lucida: Set up the drawing board up at an angle of 40 degrees; putting it on your lap and resting it against the edge of a table works well. Put a piece of paper on the board, up to A3 in size. Swing up the arm with the 'viewing lens' up, twist the 'lens' so that the small eye hole is at the top. When you look through this, you should be able to see the whole piece of paper and the scene as if reflected on it.
What To Do if You Can't See the Piece of Paper or Subject on the Paper: Check the position of the camera's viewer. Are you looking down towards the paper? If so, it's a question of getting the balance of light between your subject and the paper right. Place a piece of black paper on the drawing board; if you can now see the subject, you should light it more. If you can't see the piece of paper because the subject's too strong, use a lamp to throw a bit more light on your paper. At times you'll find there are parts that are too light or too dark to see detail; you could fiddle with getting the light balance just right, or simply use your other eye or look up at the actual scene to see what's there.
Take a look at the kind of results you can expect...