For some reason people think that they need to assign fighters/warships to starbases or something. Pfft.
1. Build starbases.
2. Build ONLY starbases. Influence starbases, economic starbases, heck even a military starbase here and there, it doesn't matter.
3. Get starbase fortifications and fortify them.
4. Watch the AI suicide their fighters into them. Laugh.
Why it (kind of) works:
1. Economic starbases provide a benefit to your economy (so that you can spam out more constructors and build more starbases).
2. Constructors are cheap as heck compared to military units.
3. AI prioritize starbases, so if you fortify them enough, they'll suicide units into them non-stop.
4. Influence starbases will eventually take over the AI planets so it's a powerful offensive tool.
Eventually you win through turtle-ly attrition. If you can trade 1 starbase with 10 constructors for 15-20 level IV or V enemy fighters, that's worth it. Each one of those fighters costs the same as 3-4 constructors.