A320 flight crew reported low altitude alert from ATC while on approach.

Narrative

Being vectored for visual approach. ILS out of service. Used RNAV RNP as a reference (did not fly the approach- just used vertical guidance for reference on the descent to runway). Unfortunately we did not go direct to an approach transition and arm the approach prior to descent. After being cleared for a visual approach we reverted to flawed habit patterns and set the FAF altitude of 2000' in MCP and initiated descent. Big threat was the FAF crossing altitude was 1000' not the normal 1500'. We failed to brief and then mitigate that and subsequently got a 'low altitude warning' from Tower controller. Lowest we saw on the RA approximately 2mi from ZZZZZ was 800'