Become council of a coven. Some covens, like spellcasters, automatically promote you. Once council, click the link to your covens home page. Up at the top next to the priest/ess's names you'll find a link to the spellbook. From there, click the click to the spells and the underlined section that says "add a spell'.
You can mail the priest/ess or council of a coven to ask them to submit something for you, if you are not council.
You can write spells and suggest them in the spell suggestion forums.