USPTO website explains the patent search step. This may be helpful for the inventors, who normally do the search only using keyword search, here you will be able to perform by using classification search.
Brainstorm keywords related to the purpose, use and composition of the invention.
Look up the words in the Index to the U.S. Patent Classification to find potential class/subclasses.
Verify the relevancy of the class/subclasses by using the Classification Schedule in the Manual of Classification .
Read the Classification Definitions to verify the scope of the subclasses and note “see also” references.
Search the Issued Patents and the Published Applications databases by “Current US Classification” and access full-text patents and published applications.
Review the claims, specifications and drawings of documents retrieved for relevancy.
Check all references and note the “U.S. Cl.” and “Field of Search” areas for additional class/subclasses to search.