A note on plumbers acid flux (Bakers fluid) yes it does work however don't leave any instruments/equipment you value in the vicinity of the soldering
process because it will be as rusty as hell the day after.
activities soft and silver
1 cleanliness is paramount
2 correct solder and flux
3 temperature or your iron or gas torch
4 (soft) only apply solder when your work is at temperature
5 (silver) apply solder when your work is at temperature and just before make sure your solder is introduced to the flame and is fluxed .
4 finally use a minimal amount of solder it reduces number 5
5 clean the joint
PS if silver soldering
before the job has totally cooled quench it in cold water, this will make the flux easier to remove otherwise it hardens like glass