How To combine multiple shots into a seamless whole
Step 2: Merge Images and Background
When you’re using the Transform command, make sure to hold down the Shift key to maintain your object’s proportions.
(CLICK FOR FULL-RES)
Use the Move tool (V on your keyboard) to drag the first object (here, a motorcycle) onto the background layer. Go to Edit > Transform > Scale. Drag a corner handle and resize the object until its scale jibes with the background. (You’ll fine-tune later.) Then select the Burn tool (O), choose a brush of appropriate size and hardness, and burn darker edges to blend with background. Repeat with all composited objects.