Colonizing Sol:
1. Drop Infra on Luna and Mars, 500 is a good amount to start.
2. The civilian shipping line will start hauling colonists to them.
3. Earth will produce more than enough Trade Good Infrastructure to keep both colonies growing so you don't need to worry about anything else.
4. Let them grow to 100-500 million people before switching them to Stable (from Destination) in the Civ/Team tab as that means their Trade Goods production is high enough that you're getting nice trade income in addition to taxes.
5. Terraform first Luna, then Mars - focus on getting one done completely before starting with the other.
6. Once they are colony cost 0, they will start growing fast on their own, but I wouldn't turn them Source (from Stable) until they are over 1 billion or bigger.
7. Once at least Luna is CC0, you can expand to Mercury. Once Mars is done, you can expand to Europa and move terraformers to Mercury.
8. Rinse & repeat, the point is that you never have to build Infrastructure yourself, as the civilians will produce it for free and ship it over, creating wealth for you, and once colonies are CC0, their old Infrastructure will be moved to new colonies, allowing the newer colonies to grow faster - but you don't want to throttle this process by creating too many colonies at once. You can fiddle with Destination/Stable to finetune things a little bit but only once a colony has 25 million people or more. Exact numbers depend on how big your Earth is and how active your civilians are.
Colonizing outside of Sol:
1. Drop DSTS on future colony sites. Four is a good start, ten is better. This will make sure that there are no surprises lurking in the system.
2. Drop 2x GAR battalions as well. They can squash any unrest in the colony from overcrowding and lack of PPV in the system. Outside of Sol, these can be harder to deal with.
3. Decide what type of colony this will be:
- Earth-like, or to be terraformed
- Limited by infrastructure
- Special Underground or Orbital
4. Bring in enough colonist-holding stuff as per objective decided above
5. Civilians will transport colonists though for special cases you need government colony ships.
6. Bring in your PPV assets: FlaK barges, Orbital Weapon Platforms, Meson PDCs, System Patrol Ships, Planet-based Fighters and so on.
Basically, you need colonies for 3 things:
- Pop growth as smaller colonies grow faster than bigger ones, though massive ones grow massively even with tiny percentual increases.
- Source of wealth through taxation, trade goods and travel.
- Source of cheaply mined minerals since regular Mines are the cheapest method of mining.
And a possible fourth reason is for RP, meaning Role Playing. All colonies need security and security requires awareness. Deep Space Tracking Stations are the cheapest and easiest way to have awareness of a system. Garrison Battalions are the cheapest way to keep the colonists in order. Ship Bureau has plenty of examples of the kind of things I mentioned at point 6 above. You want security because colonies are important investments and you don't want to lose them to a hostile neighbour coming over for a visit.
Colony Cost 2 or lower planets can be easily terraformed to Earth-like so the colonists won't need Infrastructure. Colony Cost from 2 to 5 can usually be terraformed but it'll take a longer time, whereas CC above 5 can still be feasible but you should think long and hard if you really want to waste the time and effort to do it. Do note that Terraforming can be its own end-game goal, like turning Venus into an Earth-like planet.
For special cases, you can use Orbital Habitats or Underground Infrastructure. OH are good for planets were Colony Cost is ridiculous, like Venus at 25, because the higher the CC, the higher the requirement of environmental workers to keep the colonists alive - on a Venus colony, you will never have enough workforce to be able to actually DO anything until you terraform it. UGI is for places with too low gravity for Human tolerance where you still need to have actual people live & work. To be honest, I've only ever used OHs for vanity projects and I've never actually used UGI since Asteroid Miners and Auto-Mines are far less of a hassle and not that expensive.
So there is no need to commit to an expensive and massive terraforming project or to build loads of OHs since it's usually cheaper and definitely easier to put down a Mass Driver and five hundred automines.