Hello,

Do you know how to say:

Shake well before using.

...in Japanese?

No?
Okay, I will show you.
Here:

English:
Please shake well before using.

日本語：
使う前に、よく振ってください。

使う＝use
前に= before
よく= good/well

振って 「テ-form of 振る （ふる）」= shake 

ください= please

めっちゃ楽しいでしょう!

There you go!
Enjoy using it.