I can't tell you how to write them but I have wrote a few myself. How I do it is I write it as if it was a regular story but when it comes to the sex part I become more detail and graphic.

From ehow, one of Answerbag's partner sites: http://www.ehow.com/how_2101968_write-erotic-short-story.html Instructions Step 1 Figure out what the story is that you want to tell. Is it fiction based on real life or someone else's story? Is it pure fantasy? Step 2 Whose voice is telling the story? Is it one of the characters or is it someone removed? Write a brief description of the narrator -- what do they do for money, where do they live, are they funny, serious, creepy? Step 3 Erotic fiction is about physical pleasure. What words arose physical pleasure in you personally? Write them down. Are they words everyone uses? What less common words give you physical pleasure? Step 4 If you feel stuck, write down the first sentence that comes to you, like this one of mine: "So Creamy asks me, 'You gonna finish that torta, Gloria?" Just sit with that sentence. Who is Creamy? Who is Gloria? Why are they talking? Where are they? What do they really want? Start writing, see where it goes. Step 5 You will censor yourself as you write. This is very common. But, remind yourself that the physical act of drawing words on a piece of paper is not the same thing as doing what you are talking about. Where you feel least comfortable is precisely the boundary to explore in erotic fiction. Your agony will likely arouse the reader. Share it. Step 6 Give your story "multiple orgasms." Not ejaculations necessarily, but orgasms. Build up to something erotic early on, cool down, then start up again. Seduce the reader into wanting to go further. Or, if you're up for it, follow the typical pornography narrative arc: give the reader something to enjoy, and if they keep reading, make the characters suffer some setback which requires them to keep at it, taking things further than before.

With erotic words

Just like writing any other story but you use words like penetration or sweaty, and you avoid words like manboobs and fungal infection.

just have a fantasy about someone and make it perfect to your liking. then write it down.

Try this - His strong manly hands probed every crevice of her silken femininity. Their bodies writhing in sensual rythem as he thrust his purple headed man warrior inside her quivering mound of love pudding. Hope that turned you on ?