I want to crop an image, but I don't know how this works.

[Gerhard Huber]

First of all you have to create a lasso around the area that you want to crop.
Then you press "Crop" in the Tool Settings or in the context menu. You can
create a "Soft Edge" with the corresponding setting in the Tool Settings.
If you hold Shift while cropping, the cropped area will be shown in a new
window.

[emri]

2013:

Using the RECTANGULAR LASSO tool (shortcut: L), select the area you wish to crop. Then hit SHIFT-R and you're done.

Apparently you can crop with any of the LASSO tools EXCEPT the FREE LASSO.

Using the SHIFT key with the CROP tool, RECTANGULAR LASSO, and CIRCLE LASSO restricts your selection to equal width and height (i.e., square).