To select an area In the Toolbox, select the Lasso
Tool, click on the image and draw a contour holding mouse's button
pressed* (or simply draw if you
use sylus).
To close the contour just release
button if you draw with mouse or uplift stylus.
While New selection
option choosen on the Optionsbar
by creating new selection you delete existing one.
To add an area to existing selection you have
either to choose Add to selection
option on the Options bar or to press (and hold)
[Shift] key while selecting.
To subtract an area from existing selection
you have either to choose Subtract from selection
option on the Options bar or to press [Alt]
key while selecting.
To intersect new selection with existing one either choose
Intersect with selection
option on the Options bar or press [Shift+Alt]
keys while selecting.
While New selection option is chosen and
the point of cursor is within selected area
the tool looks like this
and can move selection (ie. borders of selected area and not
image content).
While New selection option is chosen you
can also move selection using keyboard
cursor keys.
* I recommend to use stylus
if available..
** On the animation above colors
were inverted just to make selected area visible (it turns
black).
*** Selection techniques explained
in detail in The Skills.