How can I relearn to burp?

When I was young, I did it all the time. In the past ten years or so, however, I've stopped entirely, and today, when I tried again for the first time, I discovered that I don't know how. Whenever I try swallowing air, it rises up in my throat but just makes a weird sort of croak-y noise and is gone, even when I don't open my mouth. Does anyone have any advice for me?