Contact the czars and let them know that you are interested in giving a talk. Look at the schedule of upcoming talks to see what dates are available. You don't have to have a firm topic in mind to sign up!
Talks should be prepared to be about 20-30 minutes long in order to leave plenty of room for discussion and questions. Make sure the include plenty of introductory material - assume your audience has never worked in your field and has a first-year graduate level knowledge of physics.
A week before your talk send the czars an exact title (if you haven't already) and a short blurb for the website and e-mails. Let the czars know if you won't be giving the talk from your own laptop.
Talk to the czars or check the website to make sure you know what room the talk will be in. Show up at least 15 minutes early to prepare the presentation and to make sure there is no last minute room change.
After your talk, send the czars a copy of your slides so they can be posted to the website