To write to someone you've already had a conversation with before, just pick a name from your conversation list and write your message.
If you have never written to this person, use the "+" button to start a new chat. You will then be directed to research so that you can easily find the right recipient.
If you want to send an urgent message, use the exclamation point in the text box.
Here are videos that show how to write to another person.
[MOBILE]
[WEB]