In this example we are going to crop an image to a square.

Firstly open the image in the editor (File, Open):

Open up your image in GIMP

Click on the selction tool. This is the top left icon in the main GIMP toolbox.

On the selections configuration click the "Fixed" tick box and make sure that "Aspect ratio" is selected. In the box below enter "1:1" as your aspect ratio

Now select the area that you wish to be cropped.

Select the area you wish to crop.

You can move the selection by holding down the left mouse button on the selected area, then drag the selection to its new position. It is also possible to change the dimensions of your selected area, this can be accomplished by left click and holding on either the sides or the corners of the selected area then dragging the mouse.

Finally go to the tool bar click on "Image" then "Crop to selection"

Finally crop the image