Yanfei's dps is from her charged attacks. Similar to Itto or Ningguang, one builds stacks by using normal attacks (and other abilities). The stacks will give more damage scaling to her CA and provide other benefits (like reduced stamina consumption, more pyro dmg bonus from her 1st passive). She can gain up to 3 stacks (scarlet seals) at c0-c5 and 4 stacks at c6.
Yanfei's E can give stacks. Yanfei's Q can also give stacks over time (similar to one of Itto's constellations). Yanfei's Q also gives increased CA dmg bonus (so it functions like a dmg bonus buff similar to Xiao's Q). At C4 Yanfei's Q can also give a shield, which has some support abilities (like a 'tankfei' build to replace Thoma for Hu Tao). So the goal is to have her burst up as much as possible to maximize her CA dps.
Yanfei's downside is that her burst costs 80 energy, so it is recommended to run her with a battery (like Bennett) or some ER on the substats. Otherwise, one can build her like any other dps. One thing to note is that building more crit rate on her can be helpful as her 2nd passive will give some bonus damage on crit hits. 4p wanderer for the CA bonus is viable. Crimson witch is also fine.
As a pyro dps, she is better than Klee.