I just want a trim. My hair's too long.
我只想修一下。我的头发太长了。

Trim
修剪（剪得少）

My hair dresser cut my hair too short. I just wanted a trim.
我的理发师给我剪得太短了。我只想要修一下。

Can you just trim the back?
可以只修一下后面吗？

I don't want to take too much off. Just this much. Up to here.
我不想剪太多。就剪这么多，到这里就行。

Take off
剪掉

You can take off a little more. I'd like it shorter.
可以再剪掉一点吗？我喜欢短一点的。

She took off more than I wanted, but it still looks good.
她剪得比我想的多了一点，但还是很好看。

Just make sure it's not more than two inches. I like my hair longer.
一定不要超过2英寸哦。我喜欢自己的头发长点。

Just make sure it's…
一定…

Just make sure it's even.
一定要对称啊。

Just make sure it's above my eyes.
一定要在我眼睛上方。