Index (this will be updated as the series unfolds)
Part 1a: Importing Files and File Types
Part 1b: Camera Raw
Part 2: Colour:
Part 3a: The Workspace
Part 3b: Example Layers Workflow
Selection
Marquee, Copy, Paste
Zoom
Opacity
Move
Renaming Layers
Visible Layers
Opacity & Flow
Eraser
Part 3c: Adjustment Layers
Part 4: Grouping, Masks & History
Part 5: Selection and Paths (Pen Tool)
Part 5b: Healing Brush, Patch or Clone Stamp Tool?
Part 6: Image Size & Interpolation
Part 7: Smart Filters & Sharpening
Part 8: Image Interpolation & Artefacts
Example Layers Workflow:
This week, we present a step-by-step guide that will help you understand layers, as well as a series of tools along the way:
Taking two shots from one shoot, we wish to take the eyes from one and position them over the other expression. By deleting the 'offending areas' of the copied eyes and carefully blending the two, it will give the impression of an original image.
Caption: We will take the eyes from the left image and apply them to the image on the right. Note the two separate image windows, denoting that these are two separate images. The bright blue window denotes that this is the image currently in work.
Selection
Caption: Rectangular Marquee Tool selected from toolbar. Click and drag a rectangular selection, releasing when satisfied. This generates the selection as outlined above.
Edit > Copy (Ctrl+c) > Now select the other image that you wish to paste into > Edit > Paste (Ctrl+v) > This will automatically generate a new layer with the pasted contents (default paste position is central to image).
Caption: Centrally pasted image, note that in the layers window (bottom right) the selected (working) layer is bright blue. It is important to ensure the layer that you wish to work upon is selected (a common mistake is to select the wrong layer and wonder what's going on).
So far, we're under no illusion that this facial landslide is remotely complete. There needs to be a lot of tweaking to get anything resembling realistic.
Zoom
In this case, the image is a lot bigger than the screen we're working on. To display the whole image, Photoshop is showing it at an appropriate zoom level to fit to screen.
The Zoom (z key) is one of the tools that you will most use - hence that 'Z' shortcut key will be taking a continual bashing. 
To zoom in, simply click your mouse's left key. The top of the image window will show you to what percentage you are zoomed in - this can range up to 3,200 per cent, where individual pixels become supersized on screen. At some zoom stages the image may look strange, but this only occurs for display purposes. At 100 per cent zoom, however, you are seeing the pixel for pixel size of the image, so it should look decent (if it doesn't look good at 100 per cent, it never will).
Caption: Shows image is at 100 per cent ('actual size')
If you want to zoom back out again, then either you can adjust the zoom function in the display bar at the top, or simply Click and hold Alt and continue to left mouse click as before (only now it works in reverse until you release the Alt key again).
Opacity
With our image, it's tricky to see what's going on in detail - so zooming in gives a clearer and more accurate presentation on screen. Then, in order to align layers as accurately as possible, we're going to reduce the opacity of the new layer, helping to match the eyes up.
Caption: Opacity from 1-100 per cent, affects the layer selected (as distinguished by blue highlight)
Move
It's possible to move around one layer while the others remain static - this is achieved with the Move tool. Once selected, click and drag the current layer around to the designated position. Zooming right in can help with accurate matching, though Photoshop may try to automatically 'help' you do this with the snap mechanism. This is where images 'snap' into place (you see where they got the name from?). If snap is doing things you don't want it to, then it can be turned off via View > Snap (Shift+Ctrl+;) for your own placement. If you find images are often sucked to the edge of frame then this is a feature of snap also.
Scale is a problem with the two images in this example, as the model was slightly closer to camera in one frame than the other. In this instance, it is probably best to treat each eye separately - so by the same process (marquee selection tool, copy, paste to new layer) we can make two new separate layers (one for each eye).
Renaming Layers
Sometimes this is necessary when copying a selection and pasting it within the same image. As it is possible to paste multiple different selections into the same layer, Photoshop may be set up to assume that you wish to paste into an existing layer. To avoid this, it is simple to create a new blank layer to paste your content into - remember that by left clicking a layer will activate it as your current working layer. Clicking the 'Create New Layer' button in the Layers window will create a blank layer, called 'Layer #' - an uninformative title that you are able to change as you wish. //IMG 009 align to right//
Caption: Renaming layer
By hovering over the text of a layer ('Layer 01' for example) and double clicking, the text will become selected and can be typed over. Where there begins to be a great number of layers, it can become confusing as to which part is what, who or why - so the naming process becomes an important reference system.
Visible Layers
It's possible to 'turn layers off' too. Next to each layer there is an 'eye' symbol. By clicking this it will vanish, thus removing the layer from visibility. Fortunately, the layer has not been deleted and is still possible to toggle back on for use.
Opacity and Flow
To blend our example images together, simply delete the parts that you don't want to see. This can be achieved with the eraser tool in this instance. 
As with layers, many tools can be used to varying degrees of Opacity or Flow. These two features may appear to perform the same functions, but they are distinctly different. Utilising low brush opacity when erasing can cause notable issues:
Caption: Here the eraser tool has been used at 100 per cent, 50 per cent and five per cent opacity. Opacity is not the ideal feature to adjust for brushing and erasure; 'Flow' best attributes to this.
As opacity has been used rather than flow, the refresh rate of the brush may print the brush pattern, even at a low opacity (see the 50 per cent example in image).
This is why Flow and Airbrush are key to a fluid, clean brushing and/or erasure. It's likely that you've heard a fair deal about 'airbrushing' in the media. Try to clear that from your head though, as the means we're using it for here have nothing to do with Beyonce's hips.
When airbrush icon is clicked on, and if the left mouse button is held down, then the brush will continue to operate. So a two per cent flow will slowly build up to the equivalent of 100 per cent stage by stage. Whereas if the airbrush icon is clicked off, each separate click of the mouse will cause an action, holding the left mouse key down will only function as one click.
Caption: Seemingly a minor difference, but if you're using a graphics tablet or brushing with a mouse, then the progressive nature of the airbrush can be priceless.
With our example image, there are now multiple layers - the original invisible layer, a duplicate of this (as the main image), and two separate layers - one for each eye.
With each eye, here the left, carefully brush away the unwanted areas (with the flow percentage set low) to reveal the background image. It's important to maintaining the correct portion of the current image and to run along with facial contours to try and maintain realism. (See ThinkCamera's four-part Skin Retouching How To for a fuller example of face and skin retouching).
Caption: Here's what the layer looks like if all others are turned off/not visible.
Caption: Here's the final image - original layer, a copy of that layer, with 2 additional layers (1 for each eye) mounted on top. It may not be perfect, but it's an example of how images can function.
A series of simple processes all grouped together can generate a seemingly 'original' image - that's half the trick, if your adjustments or aesthetic corrections are subtle enough to look real or possible, then you're half way there!
If you take this basic concept, marry it with other knowledge that you will learn as you go on, twist it up with your imagination... and the possibilities of your work will be endless.
Continue to Part 3c and Adjustment Layers...
