Guides:
Now that you're all set up, it's time to actually use the damn thing. To start a new chat (or to join a public room/create a new room), simply click the plus icon near the top on the home screen as demonstrated by the image below.
If you click "Start new chat", you will be prompted to enter the user you want to chat with. This is typically formatted as "@username:matrix.avaisa.pro", however simply entering the "@username" should autocomplete the rest. Click the user you want to chat with and then click "Go".
Once you are in the chat with the user you want to chat to, send them a message. This will create the room with that user (essentially like a direct-message), which will likely be encrypted assuming both of you have set your accounts up properly. The user you are starting the chat with then has to accept the chat request, starting your conversation. There are call/video-call buttons at the top of the chat window, I recommend having a play with all the different buttons.
Congrats! You have now created a 1-to-1 chat with another user!