Think of an annotated bibliography like a guide to a research topic that you are preparing for someone else. A bibliography consists of a list citations to published works - which may be research articles as well as other items such as books or book chapters, patents, conference proceedings, etc. In the case of an annotated bibliography, each citation is accompanied by an "annotation" or a paragraph that provides a concise summary of the work as well as some assessment of its value or relevance, or in our case, a statement that considers how the work described in the item contributes to our understanding of the larger research topic. Generally, an annotated bibliography as a whole attempts to provide a coherent and accessible account of the research that has been done on a given topic up to that point in time.
The most effective annotated bibliographies start from research questions themselves. Such as, what is know about a certain type of chemical process? What methods have been employed to accomplish a certain type of chemical transformation and how successful have they been? Or, what has been the trajectory of a particular chemist's research agenda? Likewise, the first step in creating your own annotated bibliography will be developing a research question that will in turn guide you as you head off to search the literature.
Part of the work of making an effective annotated bibliography is taking care care when selecting the sources you will include. How you select sources will ultimately determine the quality and usefulness of your bibliography. Carefully define the scope of your own research question, and use it as a guide when deciding which sources to include and exclude from your bibliography. When considering whether to include a source, force yourself to articulate exactly how it relates to the research question you've posed. Then decide if the answer to that questions is compelling enough to grant the work a place in your bibliography.
When preparing an annotation, aim to briefly restate the main argument of the source. In order to do this effectively, you'll need to identify the research question being asked by the authors, their methodology, and their key findings. You'll also want to take a moment, to the best of your ability, to evaluate the soundness of their work. Did they find something compelling yet their methodology seems suspect? Or did they pioneer a new approach to answering a question that merits notice, despite producing inconclusive findings? In this process, consider looking beyond the article itself. You can look up the lead authors of the study and try to familiarize your self with their research programs and the types of questions that animate their research agendas and approaches to the field. Try, the best you can, to become an expert on that paper and represent it candidly and respectfully.
Then, you must assess how each source informs or contributes to your own research question. This will involve making a statement about how the work itself contributes to our understanding of the topic.