For an overview, have a look at these videos and guides, which give you search strategies that you can apply to any database.
Step 1: Use a question framework (such as PICO) to formulate the search strategy.
Clinical Question: "In patients with hypertension, will garlic extract effectively reduce hypertension?
Patient problem/Population: hypertension
Intervention: garlic extract
Outcome: reduced hypertension
Step 2: Starting with the Patient Problem(s) and Intervention, construct keywords for searching (this example is showing a search in the PubMed database)
Group similar terms together with brackets and the word OR; separate groups with the word AND. Group phrases together by putting them in quotes, for example:
(hypertension OR "high blood pressure") AND (garlic)
Step 3: Perform search, then review your search results.
Are there any synonyms you could add to your search?
We can see in the example from PubMed below, that another word for "garlic" is the scientific name "allium sativum".
Step 4: Adding synonyms increases your results.
Step 5: Now narrow your search down by selecting the relevant study design (for example, randomised controlled trials or systematic reviews). Most databases have a way of doing this. In PubMed, select the options on the left.