need help re correct sequencing of project

[denzjos]

Select the plate with the "Ellips Select Tool'  (marching ants) and press 'enter' on the keyboard
Invert the selection:  'Select / Invert' on the gimp menu
Fill the background with a color, use the 'Bucket Fill Tool'
Then 'Select / None' to remove the marching ants
Select the square you want with the 'Rectangele Select Tool' (marching ants)
Then 'Edit / Copy' and 'Edit / Paste As / New Layer
Set the original layer off