Select the crop tool from the toolbox (the paper knife icon) or click Tools › Transform Tools › Crop.
Click a starting corner and drag to outline the area to keep. A rectangle showing the crop area will appear.
To adjust the size of the rectangle, move your mouse pointer above any of the rectangle's sides or corners, then click and drag to resize as desired. If you want to adjust both width and height of the rectangle, use a corner. To adjust only one dimension, use a side. To move the whole rectangle to a different position without resizing, click anywhere near its center and drag to the desired position.
When you are satisfied with the crop area, click anywhere inside to crop the image or press Enter. To cancel the cropping, click anywhere outside the crop area.