Sufficient heat mostly, and flux.
Big stuff is difficult because the part acts as heatsink, which makes it difficult to get sufficient localized heat to allow the joint to flow. Helps to have a higher wattage iron or a soldering gun for bigger metal parts, capable of getting enough heat in quickly enough to make the joint before the entire part heats up fully. A smaller iron which works great for small individual wire connections may struggle to heat the entire part or may not work at all for something like a shield ground connection to the connector housing or heavy gauge copper connections for example.
Other that that, basic thermal dynamics stuff always applies-
Heat rises so place the iron underneath the area to be heated rather than on top.
Wet the iron sufficiently with a blob of fresh molten solder for efficient heat transfer from iron to part.