Select studies for inclusion based on pre-defined criteria. Start with a title/abstract screening to remove studies that are clearly not related to your topic. Use your inclusion/exclusion criteria to screen the full-text of studies. It is highly recommended that two independent reviewers screen all studies, resolving areas of disagreement by consensus.
It's easy to get lost doing research. Keep that inclusion/exclusion criteria close so that you are better able to decide yes or no on an article. If it's a great article but doesn't fit, email it to yourself and read it later.
We also find that it is important to know what to say, "I quit". Sometimes that decision is made for you (i.e., you have two weeks to do this search or you have six months and after that, that's it). Other times, you have to know when to stop. Are you seeing the same articles over and over? Have you done a comprehensive search in other databases? It's okay to stop.
Evaluate the risk of bias of included studies. Use a Risk of Bias tool (such as the Cochrane RoB Tool) to assess the potential biases of studies in regards to study design and other factors. You can adapt existing tools to best meet the needs of your review, depending on the types of studies included.