While brass/bronze is more forgiving of poor prep, cleaning and lack of flux and many just want to get the job done as fast as possible, these are no reasons to not follow best procedures. So most here will suggest that you prep well. Clean by sanding fully down to bright surfaces. Have your fit up as it needs to be, not almost right. And use flux on all brazing surfaces before starting to heat up. Powder won't stick to steel unless the metal is hot, and then the oxidation that flux helps prevent has already started. paste will stay put while heating up and by the time the water steams off the resulting powder will stick and do it's job. Which flux can be debated but many find the GasFlux company type B the best for brazing and clean up. I'll dip my warmed up rods directly into the paste as i feel the need to have more flux on the joint. I do have generic powdered flux and might sprinkle some on (just a pinch will do you) fillets if the flux is getting "old". As you get more proficient you'll find aspects that you can short cut but when starting do the full thing. Andy.