Summarize this article:

Detailing clay (or glass cleaning clay) is an elastic compound which can capture grit and grime lodged within a crevice and pull it out.  If your car’s windshield has any dings in it, dirt can accumulate within.  Even if there are no visible indentations, the fine particulate matter which has accumulated across the surface of your windshield can be removed with detailing clay.  Obtain detailing clay at your local auto care shop. Each detailing clay has specific directions for use.  Carefully read the package and follow the directions. Next, apply an automotive lubricant to the windshield.  This liquid combination will help the clay glide across the surface of the windshield.  The amount of each you’ll need depends on the size of your vehicle.  A bus will require a good deal more water and lubricant that a small car. Grip it in the manner you would a bar of soap.  Keep your middle and index fingers on top of the clay, your thumb on one side and your remaining fingers on the opposite side.  Move the detailing clay through the lubricant/water you’ve applied to the windshield.  The clay should glide back and forth smoothly across the wet glass. Reach over the windshield and place the clay in the center.  The clay should be placed in the center bottom of the windshield where it meets the hood. Bring the detailing clay from the base of the windshield to the top where it meets the roof.  When you’ve completed one vertical line, place the clay at the bottom of the windshield again, but in a location slightly closer to you.  Draw a second vertical line up from the windshield to the roof of the vehicle.  The second line should be parallel to the first.  Continue to drag the clay across the surface of the windshield in straight, vertical lines, moving the rows gradually closer to you. When you feel the clay slow or stick in its motion across the windshield, you’ll know it has encountered some grit or grime on the windshield. Start again by placing the clay on the the bottom center of the windshield.  Move the clay up in a straight vertical line toward the roof of the vehicle.  After completing a vertical line, bring the clay back down to the base of the windshield.  Position it at a point slightly closer to you, but directly next to the location you originally moved the clay from.  Continue moving the clay up the length of the windshield in straight vertical lines, gradually bringing the clay closer to you. Take a microfiber towel in one hand and move it across the windshield in broad, circular motions.  This will remove any excess clay which may still be attached.  You can choose to use either the same hand or a different hand on the two different halves of the windshield.

Summary:
Obtain a 3 or 3.5 ounce bar of detailing clay. Mist the windshield with water. Grip the detailing clay lengthwise in your hand. Position the clay on the windshield. Move the detailing clay across the windshield. Clean any grit you encounter. Move to the other side of the vehicle and repeat the process. Wipe the glass clean when finished.