Got this question numerous times, so decided to make a mini-guide about it.
Start you message of with: /me
Continue your message normally.
If I wanted to type that you like the new PVP gear, I would write:
/me likes the new PVP gear.
It will then turn out like this once I enter it in chat:
If you still have any questions or troubles regarding doing this, just type them in the comments below.