Download and install Riot for the platform of your choice. Supported platforms include Android(Play and F-Droid), iOS, OSX, Windows, Linux, and browser.
You should see a screen similar to the below image. Enter your information and make sure that you use the default matrix.org server unless you know what you are doing. If you do not enter an email or phone number you will have to authenticate through the browser.
Submit your username to Lambdaconf to be added to the Riot community.
Once you have been invited you will see a notification like the image below.
Click on the invitation and you will see this.
Click Accept and you're in!
After accepting you should see a list of rooms like so.
Click on a room and you will be prompted to join.
Ok the warning to finally join.
You will see this message upon sending an initial message.
Click send anyway and you're all set!