This assumes that you've already enabled the IM Chat system.
Once the IM Chat is enabled, it will take the place of the PM system and be found in the same top menu chat icon:
Click to open the chat and there will be two options, Conversations and Friends:
Just click to select a friend from the Conversations or click one in the Friends list and you can start chatting.
To close the chat box, click the top red button.
To make the chat a stand-alone chat so you can make it full screen, click the Green button at the top right.
If you want to use smileys or the attach link feature, click them to use them inside your chat. Smileys only work if clicking the smiley you want to use:
Once you have one of those open, if you don't want to use them, click the drop down close arrow at the right:
