Answer by Shumail Mohy-ud-Din on Quora :
Start by looking into the project ideas page of Mixxx for year 2013 & 2014 on their website, to get an idea of their GSoC projects from previous years. For example, this page lists ideas for 2014 year:
Based on that, start to understand their codebase and try to get involved into their developers community. And when Google announces organizations for year 2015 & if Mixxx is selected, talk with the mentor, decide the project you want to work and submit your proposal. Getting involved at earlier stage (for example now) helps a lot as the developers community would already be knowing you and you’d be preferred over other candidates. Organization usually prefer candidates those, they think, would stick and keep contributing even after GSoC.
I hope that helps. Good Luck!