A synthesis is a written discussion that draws on two or more sources. It follows that your ability to write syntheses depends on your ability to infer relationships among sources like these: essays, fiction, interviews, articles, reports, lectures, visual media. This process is nothing new for you because you infer relationships all the time--say, between something you've read in the newspaper and something you've seen for yourself, or between the teaching styles of your favorite and least favorite instructors. In fact, if you've written research papers, you've probably already written syntheses.
The first step to synthesizing sources is being able to summarize them. At the same time, you must go beyond summary to make judgments--judgments based on your critical reading of your sources: what conclusions you've drawn about the quality and validity of these sources, whether you agree or disagree with the points made in your sources, and why you agree or disagree.
Because a synthesis is based on two or more sources, you will need to be selective when choosing information from each. It would be neither possible nor desirable, for instance, to discuss in a ten-page paper on the American Civil War every point that the authors of two books make about their subject. What you as a writer must do is select from each source the ideas and information that best allow you to achieve your purpose.
https://www.theatlantic.com/national/archive/2012/06/moneyballing-criminal-justice/258703/
https://www.nytimes.com/2011/12/02/opinion/magazine-global-agenda-now-more-than-ever.html?pagewanted=all&_r=0