You should plan to search at least three literature databases.
Journals that publish systematic reviews most frequently require searches of:
Searches MUST be reproducible and exhaustive. Truly exhaustive search strategies combine subject heading and keyword search strategies. A good search strategy is both sensitive and specific. Sensitivity is the ability to identify all the relevant studies and specificity is the ability to exclude irrelevant studies. As you build your search strategy, you'll need to strike a balance between being comprehensive and maintaining relevance. See Cochrane Handbook Designing Search Strategies
Issues to consider when creating a systematic review search:
"Citation chaining" is a way of searching both backward and forward in the literature to find more relevant papers using a single paper as a starting point. Such a search, starting with one paper, creates a "chain" of references linked backward and forward from the original paper.
How does it work?
When you have a relevant paper, see the references it cites (in the cited works/references/bibliography). From those, there may be other relevant papers that those papers cite that may be of use. This is going backward in the chain. One can go back many times in the literature.
To go forward in the chain, see what other papers cited the original paper.
To do that, here are two recommended resources:
Google Scholar: In Google Scholar, search for the original paper. In the results, look for the "Cited By [# results]" link just below the link to the paper.
Web of Science: In Web of Science, use the Cited Reference Search.