Your question states how to add a source, not how to write a source.
So the easiest way to add a source is to put the source between the two tags <ref> and </ref> when editing a profile.
Fact 1.<ref>Fact 1 source</ref>
Fact 2.<ref>Fact 2 source</ref>
etc.
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Also, you need the tag: <references />
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Also, every profile requires (as a standard) these three items (in this order):
== Biography ==
== Sources ==
<references />
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Putting it all together you have:
== Biography ==
Fact 1.<ref>Fact 1 source</ref>
Fact 2.<ref>Fact 2 source</ref>
etc.
== Sources ==
<references />