On mobile phone
1.Go to the profile of the person you'd like to send the message to
2. Go to the bottom on the screen and click on the paper airplane (highlighted below):
You will then be taken to a screen where you can type in your message.
On computer or tablet device
1. Go to the profile of the person you'd like to send the message to
2. Across the middle of the profile will be a bar that looks like this:
Click on the "Message" section and you will then be taken to a screen where you can type in your message.