The best thing you can do is begin planning in advance to present a complete and high-quality application. This means doing research to find out who the faculty are in both the School of Social Work and the discipline in which you are interested, and thinking about how their interests line up with yours. Also, prepare well for the GRE and/or TOEFL tests and leave yourself enough time for a retest if you feel you have not done your very best.
In this same vein, start well in advance contacting current and previous universities and colleges about submitting grade transcripts. We cannot consider your application without them. Finally, think about who you will ask to write letters of recommendation for you. Try to get at least two of the letters from a current or former professor. If possible, talk with the people who will be writing letters for you about your plans to apply to the Joint Program. It is important for them to know what your career goals are and why you are applying to a doctoral program.
When submitting your application, it is crucial to write a clear, concise personal statement outlining your professional, intellectual, and research interests and career goals and how they connect to the professional and intellectual commitments of social work and your preferred social science. Take time in preparing this statement, and include an assessment of how your interests connect to the interests of the faculty.