Synthesis involves collating and summarising the results from the data you have collected in response to your review question.
Present in a concise way you could base it around PICO or PEO tables you could also add the type of studies, sample size, etc if relevant.
If you have conducted a quantitative review it may be possible to do a meta-analysis to combine your results for a qualitative review, use themes and sub-themes.
Sometimes a narrative (descriptive) telling of your results may help you decide how to proceed, or if the studies are sufficient similar it may be sufficient in its self or if your studies mix and match quantitative and qualitative methodologies.