a short answer to this question is “heat”. You can draw or pull the
solder towards the hottest spot. It will flow in place and kind of
glue the ends together. Remember, it “acts” like a glue and is not
ment to be used as a “filler”. That’s why you need to file or saw
the two end as close as possible toward eachother.
I used the expression “acts”. The point is that the ends, soldered
together, should have a very small common layer of metal combined
wit the solder. This will give you a strong and reliable joint.
I hope that I explained this good enough. Anyway, flux will not
garantee that the solder will flow. the metals need to be clean. Do
it step by step and create a routine. After some time, you will be
able to solder within seconds if you do it the way it has to be done!
Anybody can do this but most craftsmen start of with the wrong
procedure and keep on making the same mistake(s)
have fun en enjoy