Synthesis involves combining the results of the studies included in the review, summarizing their findings and drawing reliable conclusions based on the quality of the evidence. Synthesis may be done quantitatively using statistical techniques, such as a meta-analysis, or through a narrative approach.
In general, making recommendations for practice does not fall within the purview of systematic reviews. This is typically the domain of clinical practice guidelines. Systematic review authors can make conclusions about the need for further research or draw conclusions about the usefulness of an intervention.
When preparing your report or article, refer to the PRISMA Checklist. Many commissioning bodies and journals have adopted PRISMA as the required methods for reporting systematic reviews.
You may also find this toolkit from the EQUATOR Network useful. It contains resources for "writing a great research paper using reporting guidelines."