Step 1: Take all the western cultural sexist stereotypes that apply to women (female) and men (male).
Step 2: Redefine "woman" and "man" in terms of all these sexist stereotypes, instead of by sex.
Step 3: Anyone who wants to adhere to the sexist stereotypes imposed upon women is now a woman. Anyone who enjoys the sexist stereotypes associated with men is now a man. Invent the term "non-binary" for people who don't want either set of sexist stereotypes to apply to them.
Step 4: Whichever category of sexist stereotyping from Step 3 an individual feels most comfortable with becomes their "gender identity".
Step 5a: Replace sex with "gender identity" in law and policy everywhere. Formerly single-sex spaces and services are no longer separated by sex, but by which set of sexist stereotypes a person feels most comfortable performing. For example, if a male human criminal enjoys wearing dresses and make-up, incarcerate him in the women's prison.
Step 5b: Replace any language that references anyone's sex with a sex-neutral term instead, so that those who enjoy performing sexist stereotypes feel comfortable and unchallenged in their beliefs. For example, replace "expectant mother" (a term implying the female sex) with "pregnant person", just in case a female human who aligns herself with the sexist stereotyping associated with men bears a child.