How to use Extensions

Hi there,

I have read something about extensions, but I have not undertood really how
can I use them, and, mainly, how can I ceate my own extensions. Could you
help me on this?
All the Info I have found is about how to use a particular extension, but
not how do they work or, I repeat, how can I create them.

Thanks in advance,
    DavidD 

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