How to replace the sky with a Sunset

Step 1.  I uploaded a nice sunset
Step 2.  Used Edit -> Copy image to clipboard
Step 3.  Upload my landscape picture. 
Step 4.  I used the Paint Bucket Tool
Step 5.  Set the fill pattern to "your clipboard" 
Step 6.  Set the "Fuzz" to 20%, it may need to be 
         higher or lower for you. 
Step 7.  Clicked on center of sky for paint bucket.