As far as I can tell, you don't! The handful of genderqueer people I know accept both sets, and switch (not mid-sentence, but within the course of a single conversation) when referring to themselves.

As Wikipedia says, you can use both forms in writing ("Il/elle est étudiant(e)."), but I have no idea how you're supposed to say it, and anyway it doesn't solve the problem of gendered language.

The only suggestions I've seen for real gender-neutrality are:

• Use "y" as a gender-neutral pronoun, from (somewhat archaic) corruptions of other pronouns ("Je lui ai dit" (I told him/her) becomes "J'y ai dit").
• End words that end in "é" (masculine) and "ée" (feminine) in "ey" for gender-neutrality. This covers a lot of adjectives, but not all.

Neither have caught on anywhere.