To create your own diff for a map, go into the osu! song editor, and click file -> save as new diffculty. Afterward, name the difficulty to whatever you want, and start mapping. When you are done, go into your osu! song folder for that song (file-> open song folder) and select the .osu file with your difficulty name. That .osu file contains all the information about your diff. If you want other people to play it, share the .osu file with other people and ask them to put it in their song folder.
However, if you want many people to play your guest diff, you will need to ask the main mapper of the song if they want your guest diff or go through the ranking process
http://osu.ppy.sh/wiki/How_to_Get_Your_Map_Ranked .