You use a wordprocessor.

	Ok you were thinking more of the stylistic etc stuff.  Naturally there's 
no set format to writing the story, basically the writer takes two or more 
characters, sets up a situation where they are together, and describes them 
having sex.
	I think I can define some of the bases of the storylines though.
	The simplest type of lemon story to plot is a fill in.  Basically in 
some Anime/Manga series there's times when we KNOW that the characters have had 
sex off camera and the story is writen to extrapalate how it probably happened. 
 EX: In Dragonball there's three such incidents, Goku's honeymoon, Trunks 
conception, and Goten's conception that week before the cell game.
	The next simplest and most common is a lemon story where the author 
simply takes two people established to be in love and creates a situation where 
they express that love physically.  EX: Ranma with Akane, Kyosuke with Madoka, 
Usagi and Mamuro
	There's the storylines where an established to be unrequited love is 
sudenly returned in a lemon situation.  It requires a great deal more 
justification, well in terms of how much any lemon needs to be justified.  EX: 
Akane Ryouga or Kuno, Ranma Shampoo or Ukyo, Aliel with Sheila-Sheila or Nanami.
	Finally there's stuff where people take two characters neither of whom 
have any intrest in each other and throw them together just for the hell of it. 
 EX: Ranma and Ryouga, Lum and Sakura, Kuno and Kodachi.