If a person is a bal teshuva (or is the child of one who has not yet set a practice) and has no particular minhag of how to do (or when) something, how can he decide what to adopt? Some minhagim are local to "Ashkenazim" or "Sephardim" but some are more geographically based within that (minhag Polin vs. minhag ashkenaz for example). Some are not tied to a place historically and are just what "some people do". Some are not even clearly minhagim that one would choose, but are presented as the authoritative "way to say it" in siddurim (though I wonder if saying things as worded in the siddur is considered minhag if the wording is different in other siddurim).
What kind of due diligence is expected of a person who is adopting a minhag? Must it make logical sense after research? Can it just be appealing on a "gut" level? Can it simply be the most convenient (the siddur I like has this so I do this)?
Note that not all minhagim are evident in the practice of a shul or a "kehillah" as advised in the related question. If the person lives away from a congregation or in a very large community and davens in more than one place so there is no single, unifying community practice. What kind of research does an individual have to do (if any) before adopting a minhag when there is no other driving impetus to adopt it?