How does it work the use of the past participle done instead of the past tense did? Where is this form used? Only in southern U.S.? How often?
I'm from the U.S. In the South, 'done' can be used instead of 'did', just like 'seen' is sometimes used instead of 'saw'. Use of the past participle form in place of the past simple form is pretty common in the South. It is also common to use 'done' in place of 'had'. For example, we sometimes say: "I done told you that" instead of "I had told you that". It is grammatically incorrect in standard English but it is part of most Southern dialects. So I would not call that slang, but rather a dialect feature. Hope this helps.
Many dialects of English, throughout the English speaking world, have different forms from the standard for the parts of verbs. "I done it" is not part of any standard English as far as I am aware (whether British, American, South African, Australian, or any other). But it is widely used in parts of England and parts of the US: I don't know about the other parts of the Anglosphere.
I would not call it slang: I would call it a part of many non-standard varieties of English.