Standard H40 Bubble Flame Psyker
 Psyker Build - Warhammer 40k: Darktide 

The basic quintessential psyker flamer build. Good damage, low skill requirements, good team utility. Making it so players entering high havoc have a baseline build, too often I see players who are built badly. This is based on mine observations of other good players as well as mine own playtime in h40. I might make one for every class. This is a more general build, you can change it to fit your needs.
 

Talents, Weapon choice and blessings
 

Dueling sword blessings: Uncanny Strike is best as always, Riposte and Empathic Evasion have huge synergy. (this can be replaced with Catachan Mk III Combat Blade with the same perks)
Inferno Staff: Blaze Away for power bonus and Warp Nexus for faster stack application. Penetrating Flame damage increase is pretty minimal on everything except carapace, and is not worth the bonus stacks from warp nexus which stack multiplicatively at the beginning, in addition stacking rending from uncanny strike is a much better way to get damage increase to dots on carapace. 

Talents: Only gonna explain the talents that i often see flame psykers in h40 missing and consequentially do low damage. Kinetic Flayer is really good at killing a special mixed in horde or picking off an elite to start spreading soulblaze stacks to a mixed horde. Be aware that it can trigger daemonhosts with dot, so be wary near daemonhosts. Wildfire is good at high havoc, it allows you to get a headstart in applying soulblaze stacks and it allows soulblaze to linger long after you've left the area on mobs if your teammates are still killing. Empathic Evasion allows you to push in ranged fire when you are flaming, and is very good for survival. Inner Tranquility  is better than Essence Harvest  for a few reasons, even though having peril reduction means it takes longer to reach the sweet spot of peril, in general you wont have high stacks of warp charge at the start of a fight as it probably decayed a bit since the previous encounter and peril reduction will help you get more flame bursts off in the tight peril zone before needing to vent, in addition you don't really need toughness as you have plenty of other ways to generate toughness when you are flaming targets such as Soulstealer .  Other important talents are Warp Rider , Warp Siphon and talents below it, Solidity , Souldrinker , Perilous Combustion , Perfect Timing . Other talents can be replaced for personal preferences.

Playstyle
 

Playing flame psyker positioning is key, as psyker its important to stay near the team but also be in a position where you will not get flanked. Your bubble is a good way to nudge your team to a specific area, whether to get them to go aggressive on gunners or to retreat to safety. Try to spam out your bubble as often as possible but be very aware of where you place it as you can lead your team into an unfavorable position with bubble. In general you want to stay between 70-95 peril to maximize the damage of your flames when you are flaming. You can give your dot rending by swapping to melee and stacking uncanny strike for things like crushers. 

For more info and testing on wildfire, penetrating flame or other inferno staff quirks i would recommend looking at Meowbeep's video on flamer staff misconceptions.  https://www.youtube.com/watch?v=4Ls2-YjlqO8