Look at the conference call long and hard. Usually the call will include a number of possible subtopics posed as questions or a list. Think about which of these fits closest to your interest, then do a bit of thinking squarely about how it relates to your existing research interest.

Look at abstracts from years past, and if the conference archives papers (as does IASPM-US on a limited basis) it is good to see how widely the abstract and actual papers can diverge. Some conferences require that you submit your papers in advance to the moderator or to a "responder" who will synthesize all the panelists' work and respond to all before the conference talk is opened for questions. That said, most pop music conferences I have gone to do not have responders, which means that you can veer pretty widely from your abstract (and, for better or worse, not have your paper done until very near the time it is read).
More traditional academic associations tend towards using commentators. That is, conferences organized by the American Studies Association or the Organization of American Historians or the Modern Language Association will more likely than not include a slot for a commentator on most panels.

Then write a draft of a the abstract. The final version will be about 250 words but in draft form can be longer. Get everything you would want to say in there. You can think of it as a pitch to a particularly smart editor, because that's what it is. You can also think of it like your hit single off your first album. Make sure to include not just the argument, but the kind of evidence you'll use (interviews, your own experience, theories or specific texts), and how you might be presenting it (showing video, playing tunes, etc). If you have a hook, and that hook can be recognized for what it is without all your supporting evidence, be sure to include that hook in your abstract. Remember that most conferences give only 20 minutes for talk, and maybe 10 minutes for question. That means you only have 9 pages to develop your argument and present your evidence. That's not a lot of room, so don't expect that you can tell a whole history, describe a whole life, walk an audience through a whole scene. Pick something intriguing, controversial, unique, new, unheralded, or otherwise brilliant about your giant topic and stick with that. There's time for the rest in your amazing book or dissertation, or blog post.

Once you do write a draft you can show it to folks you trust and get advice, fine tune and keep doing more focused research on specific people you'd want to include or research you'd want to do to make the argument really work.

Once you get your feedback and feel happy with what you've written, wait a night, reread 10x and send it off. They'll get back to you within a month or two or so to let you know if you've gotten in. You might find yourself waiting longer for a response if the conference is a large one. Don't despair. You have not been forgotten.