1. Convert the photo into a square image
Use the Photo|Image Size menu item to change the image size into a large square. Uncheck ‘Constrain Proporties’ and set the “height” value to the same value as your “width” value.
2. Turn image upside down
Rotate by 180°
3. Apply the polar effect
“Filter|Distort|Polar Coordinates” filter - choose the “Rectangular to Polar” setting
When selecting a photo to start with you should keep the following things in mind:
Select a panorama photo or a cropped landscape photo (aspect ratio at least 2:1, which means the width should be at least two times the height).
The bottom area (bottom 25% or so) of the photo in most cases should have only very little detail (e.g. sand, asphalt, water). This area will later be in the middle of the picture and will also be distorted the most.
The upper area (upper 25%) should also have only little detail, most preferably just one color (e.g. blue sky, night sky etc.).
The left and the right border of the photo should neatly fit into each other (which is always the case for a stitched 360° panorama).
The horizon must be exactly horizontal, the left and right end of the photo will be joined together and if they are on different heights your planet will have a big crack in the surface (again this is no issue for 360° panoramas).