START: Assets have no sense of conforming to value structure; no cast shadows emanating from assets.
GOAL: Assets have internal shadows and highlights which help describe their three-dimensionality and conform them to value structure of background; cast shadows are present and aligned with directional light source.
BURN TOOL: Darkens. Be sure to manipulate 'exposure' (strength of tool) and 'range'. Can be used for shadows and cast shadows.
DODGE TOOL: Lightens. Similar to (but opposite of) Burn Tool. Use for highlights.
LAYER/DUPLICATE LAYER: Duplicates current layer for creation of specific-shaped cast shadow.
IMAGE/ADJUSTMENTS/
HUE/SATURATION: pull lightness all the way down after duplicating layer to turn layer black. Once details are gone, run Hue/Saturation again and pull lightness up a small amount (to turn it dark gray). This way you can use....
COLOR BALANCE: to shift color temperature of the shadow.
LAYER OPACITY: pull down part-way to make cast shadow transparent.
EDIT/TRANSFORM/
FLIP VERTICAL: use to turn cast shadow upside-down
PERSPECTIVE: use to 'tilt' cast shadow in desired direction
WARP: use to re-align pieces of shadow after manipulating
SMUDGE TOOL: use with cast shadow to blur/fuzz up edge, align shadow with source, and/or account for shifting terrain
ERASER TOOL: use with low hardness and low opacity (in brush settings at top, not in layer menu) to 'fade' cast shadow
The following video may be watched in its entirety, paused and followed step-by-step, skimmed our jumped through, or skipped altogether, depending on your familiarity and comfort level with getting from the 'START' to the 'GOAL'.