Hover over the corners of the object boundary. The cursor will change to a scale icon. Click and drag to scale proportionally.
Hold ⌥ Option to scale from the object’s center.
Hold ⇧ Shift to scale non-proportionally, using the opposite corner as the pivot.
Hover over the middle points of the object’s edges. The cursor will change to a scale icon. Click and drag to stretch the object along that axis (non-proportional scaling).
Hold ⌥ Option to scale from the center.
Hold ⇧ Shift to scale proportionally using the opposite corner as the pivot.
Hover over the edges of the object boundary (not at the center points). The cursor will change to a skew icon. Click and drag to skew the layer along that axis.