Sorry, there is no one guide as this cannot be done.
There are very different types of contents into xpi-files (if look from the KM point of view).
Well, I'll try.
1) Any xpi itself can be installed in newest builds (at least in last KM75 beta versions - b2, b3) just like in Firefox: directly from the Mozilla Addons pages or from file saved to disk.
You control all this at about:ad
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Extensions