This is an important tutorial because it is introducing you to the quick mask. The Photoshop quick mask is a powerful way to get great selections accurately and it's very flexible.  First start off with the lasso tool

and make a very loose selection around an object

Now that you have made a selection, you can view it at any time (and do further editing) in the quick mask mode. To enter the quick mask mode press the icon as shown right here (you can also toggle between regular and quick mask modes with the �Q' keyboard shortcut).

When you enter the quick mask mode you get a rubylith. This shows that areas that are de-selected in Red and the areas that are selected as untouched or clear.

You can further edit your selection in process (the quick mask becomes a temporary channel in the Channels palette and is gone when you exit the mode) by a number of ways including selection and filling (combined), gradient and using the brush tool (most common). Select the brush tool while you are in quick mask mode.

You can change the size of your brush in the brush dialog box in the options bar. Basically what you're going to do is �paint in' the areas that you want to deselect.

In order to do this you also have to understand that black must be the foreground color in order to paint in red (don't let this confuse you...if the rubylith was black you wouldn't see anything) to deselect. To RESELECT you use white as your foreground color with the paintbrush. This is a very important concept I refer to often:

Black hides pixels. White buys them back.  It could take you a long time to get comfortable with this in an operational proficiency. I cover dozens of tutorials on quick masking and getting great selections in my Basic Photoshop DVD Training; this will help you fully grasp and know how to use all of these tools like a pro.

Also, when you choose any shade between white and black you will get different levels of opacity in the resulting selection. The closer to black you are the more invisible the pixels and the closer to white the shade of grey, the more visible the pixels will be.

White is pure original pixels ("buys back" your pixels)

 

and black is pure hidden (masked) or invisible. 

  Because the quick mask is essentially a grayscale alpha channel, no other colors are involved, just white to black and shades of grey in between. These are what you will �paint' with in order to select or deselect.

Now just take your brush with black as the foreground color and fill in the rest of the sky carefully.

   

The quick mask (or layer mask) rubylith just shows you the actual job that you are doing. Feel free to lower the size of the brush to get into smaller areas (use the bracket keys [ ] to make the brush size larger or smaller).

If you accidentally (or purposefully) spillover and deselect an area with red use the shortcuts of D,X to switch and get white as your new foreground color. With white remember you are buying back pixels, or preserving them in the ultimate selection, so just eraser the red rubylith to where it spills over into your mountain or selection object.

It might take a few minutes depending on the type of selection job and complexity (even for pro's) but learning this 'quick mask' method is really a time saver. Try using the rectangular marquee to get a selection like this (yikes). Remember that you can also combine other selection tools and you can use the gradient tool, making selections in the quick mask mode and then filling with either white or black or your shade of grey. You can also enter the quick mask mode from scratch without first making any selection (not recommended) and then you can use a large brush to just start deselecting areas to close in around your selection with black as your foreground color.

Press the Edit in standard mode button

or Q to exit quick mask, delete the temporary channel (which will return once you enter again) and see the selection job that you have created.

You can always enter quick mask mode again to keep working on or editing your selection. I much prefer the quick mask mode personally than to edit selections by making them into work paths and using pen editing tools. With the brushes it usually gets a great selection every time with nice clean edges.

Here is the view of the quick mask mode in the Channels palette where the temporary channel (alpha) is created. Note that the black area is deselected and the white area is the �selected' portion. The rubylith simply allows you to view your selection / quick mask job in progress.

Remember that you can also right click in standard mode (with selection tools such as the marquee...) to choose make work path. This is covered in another tutorial and is another (harder) way of editing your selections/paths.