How do deaf people learn sign language if they can't hear what each sign means?

How do you teach them to read if they have no idea what the words sound like or can't hear you explain what they are or mean?
How did Helen Keller learn to read braille if she had no way to find out what each set of bumps meant?
You can't tell her because she can't hear you and she can't see written words so, she can't associate a set of bumps for a written word or even a single letter.

How do deaf people learn sign language if they can't hear what each sign means?
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