There is no particular "template" for creating an AU of your own. As Zephyr says, just look at what others have created and go off that. Keep in mind that most of us have really had enough of the "20 carrier / 200 battleship / 10 million population Pacific island" AUs. You will probably get a lot more help/respect if you create something small, manageable, and (above all else) realistic, such as said 10-million population Pacific island operating a small force of coastal minesweepers, patrol boats, and a survey ship or two - oh, and said ships don't have any fuel available to leave the docks.
An AU that throws caution to the wind and produces extremely detailed and high quality giant battlecarriers and submarine battleships would be well-received, though, as long as the drawings are nice and no pretense at realism is assumed.
I must say that my least favorite type of AU is the one that generates a wall of text and no drawings, though. Just something to keep in mind. Good luck!