Stacking multiple images on one canvas in Photoshop

Create a canvas in photoshop - about 800 wide, 3000 high (to start) 72 dpi...to do this do "file new" and you will see the menu pop up
Open the scans and images you want to stack
Go to "window" and make sure the layers window is open. Each item you drag onto the canvas will create a new layer, much like a stack of paper that you can move around and re-order.
Using the Move tool (the arrow, upper right on the toolbar) click on an image that is open next to your blank canvas. Without releasing, drag it onto the blank canvas. You will see it appear as a new layer.
You can resize it down (not up) by the command Edit Transform Free Transform (or just hit 'control' 'T') and dragging on the handles that appear around the image. Hold the shift key as you drag to maintain proportions. You can move this image around, center it, etc.
Repeat for image 2,3,4...
Use the crop tool to crop unwanted canvas
Use the Image/Canvas size command to add canvas (make sure you have white as the background color)

Tip: When you have the Move tool selected, one of the modifiers you want to have checked is "auto select layer". That way, you can click on the images you have stacked and the layer the image is on will be automatically selected.