By diving headlong right into it.
No, not trying to troll, but seriously, you will not get better at anything unless you do it or make the attempt; which in this example, is writing.
I never did any of the former mentioned above, I just jump right into roleplay when I was younger.
Of course, I guess it helps having a decade or more experience of writing under ones belt(presently); but, we all have to start somewhere.
Also, somewhere back in that day, I had to write smut from scratch; it wasn't easy as I was in a similar boat. But I winged it. Sucked at it. Continue at it. Improved at it. Such things fall under all factors in life, we are never good at anything starting out.
I am also sure there are plenty of people here that will help you in partnering, if you are aiming to add erotic flavor to your writings; but remember, you will not get anywhere unless you put forth the effort.
Edit: Also note you have to have a desire to do whatever writing you are doing. You should never feel forced into anything that you may not like doing or want to do. There are many things I don't like writing about, so in general it just falls down to the individual and their preferences. If erotic writing is something that interests you, just go for it. *Shrug* And keep going at it till you get better, it takes time and effort.