To begin, I like to work on the copy of my image. Even though Picasa maintains your original picture and allows you to revert your edits, for this effect I want to keep both the original and edited photo in my library.
From the library, select the photo you want to work with and go to Edit > Save a Copy. Picasa will automatically append a number to the end of the filename and add the copy to your library. You will need to scroll to the end of the folder to select the copy of the photo.
Double click on the picture to enter view and edit mode.