The most valuable resource for conducting and writing systematic reviews is the free, online Cochrane Handbook for Systematic Reviews of Interventions. This provides detailed instructions for the entire process of conducting a systematic review that compares an experimental intervention to other interventions or controls.
Cochrane offers extensive training materials and opportunities on their training site an introductory online module, a newsletter, and workshops offered around the world. Many of their workshops are available as webinars as well as in-person. For the process of grading articles within the review, Cochrane Training offers slidecast presentations on how to GRADE the evidence in systematic reviews and create Summary of Findings Tables, based on the GRADE methodology.
Cochrane also provides resources on reviews of diagnostic studies. A Handbook for Systematic Reviews of Diagnostic Studies is being created.
There are methodological quality reporting standards and assessments for systematic reviews. There is variation of quality among reviews, like any other type of research study, and it can sometimes be useful to do an "umbrella review" (a review of reviews).