だいぶ髪が伸びてきてしまったので理容室に行ったのですが、英語でどのように注文すればよいのか悩みました。 結局色々と表現を下調べしてから行き、私の場合はこんな感じのやりとりでできましたので、参考までに紹介します。

Hi, I don't have an appointment. How long will I wait?
(About xx minutes, ...)
That's fine. I'll wait here.
(How would you like your hair today?)
I'd like just a haircut. I want to have it cut by around 1 inch. (= 2.5 cm)
And I don't like to clip sides and back, just cut it by scissors and thin out a little.
(How long would you like your sideburns?)
Around here. (point out the length)