In retrospect Prism, the third album from indefatigable pop icon Katy Perry, was a pretty patchy effort. ‘This Is How We Do’ is one of its better moments, though, and sees the singer doing what she does best ­— playing the role of party-starter. The understated, half-spoken vocals offer a refreshing alternative to some of the more operatic output from her pop peers; bubbling bass and sci-fi synths give the tune an edge and the hook will be lodged in your brain for days.