The xylophone chords for Roar by Katy Perry are pretty much the same as the chords on piano, so I learned it just by checking out the following videos:
The second verse of Roar by Katy Perry is here:
The main difference between playing piano and playing xylophone is that the piano obviously has a lot more keys, and this means some of the chord progressions can jump around in different keys.
But if you ignore key changes and stick to the same scale, then you should be ok.
I also like playing songs by Katy Perry on the xylophone because they are songs that usually depend on a high level of production to make them sound good, so when you strip them down to a basic xylophone melody, then you can really see that even the basic structure and notes of the song are actually really nice - and that's what the hits are actually built upon.
I love Katy Perry!