Steps for creating a square from a non square image:
1. Select 'Scale image' from the 'Image' menu
2. In the dialog, set either the width or height(the lowest one) to the desired dimension.
3. Click 'Scale'
The image has now been scaled, note how it is still not a square, just a smaller version of the existing image. So now we need to crop it..
4. Choose the 'Crop' tool from the Toolbox window
5. Click and drag the part of the image you want keep. keeping an eye on the bottom left of the window(It will show you the dimensions of the selection area)
6. When you are happy, and the dimensions are correct(Square), Release the mouse button, then click within the selection you have just created.
7. Save the image.