I like to get 6-9 constructors at a time, run them into an area, turn one constructor into a mil starbase with the most overlap of planets/significant areas I can find, and then add modules with the remaining constructor ships. The first constructor is +1 attack for the starbase for everything (mandatory before you can put defensive modules???), the second is +5 defense in whatever the primary weapon is for whatever enemy I'm trying to take out, and then after that, I start adding offensive/defensive ship assists, and then once I feel my fleets are properly supported, I'll add some extra offense/defense for the starbase as a "just in case".
Then I can just attack whatever I want and use the starbase bonuses, until the planets/whatever are finally vulnerable for me to invade.
I usually make custom constructors so I can add engines and slip in really quickly. I've found that a properly upgraded starbase is nearly impossible to lose in a battle, and you only need one or maybe two fleets in the starbase's field of influence to be able to do anything you want. Of course, make sure you take our your enemy's mil starbases too... You might have to design "starbase killer" ships to do it though...