後藤真希のアルバム名「How to use SEXY」は

sexyは形容詞なので、文法だ何だという話をすればおかしいように思います。sexyの名詞はsexinessですが・・・

ただ、大文字になってますので、SEXYをなにか固有名詞的な存在として捕らえているとしたらアリなんじゃないでしょうか。

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。

http://mm-bbs.org/index.php?showtopic=1906&st=0

モー娘。ファンの外人さん（たぶん）が、タイトルについていろいろ議論をしています。

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.

このあとも続くのですが、最後には、

Moving away from unnecessary grammar talk, because Japan has never used English well in the first place...

と終わっています。

sexyが大文字になって強調されていることを考えると、『「sexy」という形容詞の使い方』、という意味で捉えれば文法的にも合っています。