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Hmm, that's an area that I can't tell you too much more detail on. For me, it's really a 'feel' sort of thing. Personally, I like to add the decorations to accents in a melody line - peaks and valleys are great places to make variations to, for example, as well as notes that land on the beat. Everyone is different, though.
When it comes to decoration, there's almost no wrong way to do it. You say you don't see where it could fit, but in reality decorations are, by design, able to fit anywhere - because they're 100% cosmetic and not structural, there's no way to put it 'in the wrong spot'. If you're completely unable to find a place to insert the decorations then 'drop the needle' somewhere in the third section of your track and add a decoration in a random spot, see if it works. If so, great, if not then simply undo the action and try another spot. Or try some of the suggested places that I mentioned.