Subtitle in FCP

Use Video Generator>Boris(the type you like)
to write your text and the tab motion to reposition
the text exactly where you want it.
If you search google you will find standards for subtitling
and templates and even dedicated plugins (i.e.CHVsubtitle)

gig

I actually use the FCP text tool for subtitles because it opens alot faster than the Boris generator (which I use for everything else).
Go to Video Generators>Text>Text
Type in you first subtitle and put it on the top track of your sequence all the way across your timeline.
Use the controls tab>origin to reposition your text where you like it.
Use the razorblade tool to cut the track where each subtitle changes.
double click on each clip and change your subtitle.