When I am soldering both XT60s, EC3s or just plain 4mm gold connectors, I prefer to plug both halves of the connector together, to reduce the chance of heat spreading the pins. I usually use a soldering jig designed for the job, but failing that, I fall back to the Mark 1 BluTac lump, best helping hand ever!
Anyway, soldering..I pre-tin the wire to be soldered, then place the tip of the iron inside the solder bucket of the connector. Then feed solder into the tip of the iron, filling the bucket partially. Then remove the soldering iron and quickly insert your pre-tinned wire into the liquid solder. Hold the wire up for a couple of seconds while it cools, and hey presto, job done.