The most famous method.
Its use is recommended by several  STI prevention campaigns.

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The condom should only be inserted on the penis only after erection.
 

It must be rolled down until the base of the penis to diminish the risk of coming out during sexual intercourse.
 

A small space must be left empty at the top, where sperm will be collected during ejaculation.
 

After the end of sexual intercourse it must be carefully removed not to spill its contents.