To become an aircraft mechanic, you must take oral and practical tests as well as written tests. There is a fee for the test. A Designated Mechanic Examiner gives you the oral and practical test. You can get a list of these examiners at the local FAA office. The oral and practical tests cover 43 technical subjects. Typically tests for one certificate--airframe or power plant--takes about 8 hours.
To apply to take the written test, you must present your proof of experience to an FAA inspector at the local FAA office. There are separate tests for airframe and power plant mechanic certificates, as well as a general test covering both. If the inspector decides you meet the requirements to take one of the tests, you may make an appointment for testing at one of the many computer testing facilities (PDF) world-wide. You can get a list of sample general, airframe, and power plant test questions.
If you fail part of a test, you have to wait 30 days before you can take it again, unless you give a letter to the Examiner showing you've gotten additional training in the areas you failed.
You must pass all the tests within a 24-month period. The FAA will then issue you a certificate.