Corners, How do I sew in Gimp?

[Ofnuts]

• Align the two layers with a common corner (using guides will help)
  
• Use the free select tool to make a 45° selection that defines the angle to cut on the top layer.
  • Starting from the corner (having guides help here also)
    
  • Depress Ctrl for constrained angles and move to a second point in the diagonal (Ctrl will keep it exactly at 45°)
    
  • Add a third point
    
  • Go to the first point to close.
    
• Delete the triangle in the top layer with [Delete] or Edit ➤ Clear