sexy は普通は形容詞で使うから､How to use one's sexiness あたりが自然かも｡
あるいはsexyで｢ｾｸｼｰな女の人｣って意味もあるらしいから､How to use a sexy とかでもいちおう意味通るかな｡
しかし､どちらにせよ Google で全然用例が出てこないです｡文法はともかく､普通は使わない文でしょうね｡
That's probably the most horrible album name I've ever heard.
It's not a noun. It's a substantive adjective.
But a great name for a self-help book.
If anyone can teach me how to maximize my use of sexy, it's Maki. Or possibly Justin Timberlake. And when did sexy become a noun, anyway?
It's been a while since I studied this stuff, but....
A substantive adjective usually occurs where a noun has been omitted but is still implied by the adjective that modifies it. For example: "When faced with a smart choice and a dumb choice, I always choose the smart." The 2nd use of the word "smart" is a substantive adjective because it is understood to mean "smart choice," choice being the omitted noun.
In the case of both Maki and Timberlake's use of "sexy," I don't perceive an implied but omitted noun. Is there one? After listening to Timberlake's "Sexy Back," I got the distinct impression he simply uses "sexy" in place of "sexiness." Using the adjective form instead of the noun form is an interesting and possibly artistic choice, but would not, as far as I understand it, qualify as a substantive adjective. I don't know enough to explain what the Maki album title is about. I suppose she could be talking about using the word "sexy" itself, which would be correct english.
Moving away from unnecessary grammar talk, because Japan has never used English well in the first place...