One of the most useful features in Lightroom is the ability to apply tone-mapping edits directly from the Histogram panel. To access the Histogram panel, select a photo in the Library module and then switch to Develop mode by clicking 'Develop'. After a few seconds the Develop module controls will appear. The Histogram panel is located at the top of the 'panel stack'.
Along the bottom of the panel important EXIF data is displayed, such as the ISO value, the aperture, and the shutter speed. Sometimes, the ISO in particular can be helpful in estimating how far you want to push the Exposure and other settings. The ISO can impact the amount of noise that becomes visible as you make edits, depending on the camera, the lighting conditions and other factors.
Before you make any edits, you can turn on the clipping warnings to show you (on the preview) if any of the tones in your image are pure black or pure white. To activate this feature, click on the two small triangles near the top-left and top-right of the Histogram panel. These are the Shadow (left) and Highlight (right) clipping warnings. If any details in your photo turn pure black, they will be displayed as blue pixels. If any details turn pure white, they will be displayed as red pixels.
Before making any edits, it's worth noting that the tone-mapping controls in Lightroom 4 (found in the Basic panel) have changed in significant ways since Lightroom 3. The Exposure control now makes the image brighter or darker, by targeting the central 50% of tones in the Histogram. The Shadows and Highlights controls, work by recovering the tonal details in the image that fall between the mid-tones (Exposure) and the very brightest and darkest tones (Whites, Blacks). Each of these controls is now mapped directly to the Histogram in Lightroom 4.
To target a specific control using only the Histogram, move your cursor across it. As you do so, a new region will light up, showing you where to click and drag. Note that the corresponding control in the Basic panel will also light up.
It's usually best to start by correcting any global brightness issues with your picture. To do that, move the cursor over the center of the Histogram until the Exposure control lights up, click and then drag to the right to brighten your image. Drag to the left to darken it, as I did in this example. Note as you do this, the tones (represented as colored peaks and valleys) will shift to one side or the other. This is true for all Histogram edits.
Next check to see if you have any clipped shadow or highlight details. Clipped shadows show up as pure blue pixels, clipped highlights show up as pure red pixels. To correct clipped shadows or other shadow areas that are too dark, move the cursor over the left half of the Histogram until the Shadows region lights up, click, then drag to the right. To correct clipped highlight details, move the cursor to the opposite side of the Histogram, click and drag left. As you correct these areas, their respective clipping warnings should disappear and more details should be revealed.
You can finish your quick Histogram edits by setting the 'white point' and 'black point', extending the range of light-to-dark tones that are visible in your picture. To stretch the brightest highlights to the right edge of the histogram, move the cursor to the right edge the Histogram until the Whites region lights up, then click and slowly drag to the right (assuming there is a small gap between the tones being displayed and the right edge). To set the black point, roll the cursor over the left edge of the Histogram until the Blacks region lights up, then click and slowly drag left.