A thorough United States patent search will require a minimum of 20-30 hours of searching in the files of existing patents. And, this only includes patents issued in the United States.
1) Brainstorm to create a list of keywords related to the purpose, use, and composition of your invention.
2) Look up the synonyms in the Index to the U.S. Patent Classification to find possible classes and subclasses.
3) Check the relevancy of the chosen classes/sub-classes using the Classification Schedule to narrow the list to appropriate classes/subclasses.
4) Read the Classification Definitions to verify relevancy of the chosen classes/subclasses.
6) Review the documents for relevancy to your idea.
7) Using the references in the selected documents, identify additional classes.
Technically, there is an eighth step: REPEAT. This process will need to be repeated for each synonym you thought of and all the applicable terms found in your searches.