Originally Posted by TheRulesLawyer

Are you soldering with a set plugged together? When I solder just one end of plugs I find the heat will allow the pins to drift out of alignment a lot. Plugging in another empty plug helps prevent that. Not really ec5 specific as I use xt60's.
1) tin the wires
2) Tin the cups
3) Apply the iron to the outside of the cup and apply a small bit of solder to improve contact
4) when the cup is liquid, start pressing in the wire, it'll heat in no time.
5) remove heat

Works all the time for me. #3 is super important in my experience or I can wait forever for heat to transfer if the tip isn't squeaky clean.