After you research Terror Stars, how do y

Build or upgrade an existing starbase. Once you have terror star researched, when you go to upgrade the starbase, you'll see the terror star modules start showing up on the military modules page. There's like 4 or 5 module upgrades required to make it a functional terror star. They appear one at a time, in the order they need to be applied. I usually amass several constructors on a nearby, well-defended planet. Once I've got around 6 or 7 constructors in orbit, I launch them all and construct the terror star all at once. Be aware though, that applying the last upgrade to make the terror star functional is a 5000bc cost. Of course, the cost is well-justified when you destroy your first star system.

Good luck, and I hope this helps.
-GeorgeT