Go to:
dokuga.com/fanfiction
Go to the left column and select "My Stories"
Then create a new story
Major Site Rules:
Chapter stories must have a 700 word minimum per chapter
All stories must have an appropriate rating and category
Any author's notes must be under 300 words
No reader responses in your story please
To get accustomed to this fandom, what I'd recommend to you more than anything is to read some fanfiction.
Start with this thread:
dokuga.com/forum/6-general-discussion/44460-the-first-fanfic-that-got-you-hooked-to-sesskag?limit=25
There are a number of very popular stories in there that got people really familiar with this pairing. The iconic one is of course, is House of the Moon. If you're looking for something more platonic, 7FFT is a good spot to start. I feel slightly guilty about mentioning it though
Tales from the House of the Moon by Resmiranda
dokuga.com/spark/story/244/1
Seven Feudal Fairy Tales
dokuga.com/fanfiction/story/159/1