This applies to most occupations.

It is necessary for an employer to prove that there are not sufficient U.S. workers who are able, willing, qualified and available at the time and at the place where the alien worker is to be employed.

Also, the employment of such alien will not adversely affect the wages and working conditions of workers in the United States similarly employed, e.g., an employer cannot pay an alien less than it would pay a U.S. citizen for the same job, nor can an employer require the alien to work longer hours than a U.S. worker would have to work.

The labor certification process is technical and specific. 

An application is filed with the Labor authorities(not the Immigration and Naturalization Service), describing the job being offered and the qualifications of the alien.  The offer of employment must contain a complete description of the job to be performed and the minimum education and experience required to satisfactorily perform the job.  These minimum requirements are often less than those possessed by the alien.  So if the alien has 20 years experience on the job but the job could be performed by someone with 3 years experience, the employer can only require someone in the U.S. workforce to have 3 years experience.

The requirements may not be unduly restrictive e.g. a foreign language, more experience or education than is necessary to perform the job, combining two jobs into one that is normally performed by more than one worker, or knowledge or skills that cannot be obtained in the United States. 

A stipulation for a requirement, such as one of the above, may be essential to an employer's business operation and is acceptable as long as the employer adequately explains and documents the business necessity for such requirement.

The employer should post a notice at the place of employment setting out the job offer.  This is aimed at qualified employees within the organization, as well as at non-employees who may learn of the job from employees.

In addition, the employer must attempt to recruit U.S. workers through sources which are likely to produce workers and are customary to the occupation e.g. advertise in newspapers or magazines, unions, etc.

U.S. workers may not be rejected for a job opportunity offered to an alien except for lawful job-related reasons e.g. an applicant does not have the education, training or experience, bad references or lack of proficiency in English.  Personal situations such as someone's marital status, religious   affiliation or factors which can only be determined from actual job performance like initiative or interest in work are not job-related reasons for rejecting the U.S. worker.

If the Labor Certification is approved by the Department of Labor, a petition is then filed with the Immigration and Naturalization Service to have the person categorized in one of the preferences. 

It is not necessary to attempt to recruit a U.S. worker prior to filing an application for Labor Certification.  However, previous unsuccessful recruitment efforts could enable an employer to request a Reduction of Recruitment which is a "fast track" system for obtaining a Labor Certification.

The Labor Certification application is extraordinarily slow and whenever possible, the Reduction of Recruitment process should be used.