What I do is:

a) Check if the application is in http://www.transifex.net/ - most probably it's not, far to few developers use this infrastructure, but if it is: start there.

b) If it's not in transifex, download the source package, look for the pot/po-files there and start working on them with an editor (such as poedit).

c) Put the the mo-file you created on your own N900 and look if it behaves well.

d) send it to the developer or open a bug in the projects bugzilla with the po-file attached.