Lol you silly ppl I'll break it down for you.
Sorry for the length and the inevitable spelling errors
1.Get the pre-requisites for the tech
I have played many different races and as far as I can tell terror stars are available to all of them but they are at the end of different tech trees so just because terror stars became abvailable after you reasearched X tech dosen't mean they will for another race.
2. Reasearch Terror Star techs
There are five terror star techs each enabling you to build another portion of the terror star. BUT A Terror Star is a useless lump that cannot move or attack until it is finished so it is probably in your best interests to wait until you have all the techs before you build one since they are destroyed in one shot.
3. Build your Terror Star
Terror stars are built by constructors and are in the same menu as the starbases once you have reasearched the tech. just build with a constructor and then fly other constructors into it just like how you would build a regular starbase.
4. PROTECT YOUR TERROR STAR!
after you have built your Terror Star there is a ten week mobilization period where your Terror star is completely immobile and defenseless so keep a fleet handy to ward off any unwanted visitors.
5. Use your Terror Star
Once the ten week mobilization phase is over your terror star is ready to rumble but there are some severe handicaps you should remember:
-it has a mandatory movement of 1 which means it is VERY VERY slow.
-While it does have a fair amount of hit points it is completely defenseless so if it gets attacked on it's own by anything it will be annhilated.
But the one upside to this is that it can be fleeted which means you should always have it surrounded by it's own little guard to keep it protected during it's inevitably long journeys through space.
But how do you get it to attack? Simple, just move your Terror star within one space of the star you want to destroy and move into the star just like how you initiate regular combat the Terror Star will swoop up above the star and shoot it's big nasty laser and wipe out THE ENTIRE STAR SYSTEM but watch out! Because this means that it will destroy any ships in the vicinity of the star system including yours! So always remember to move your ships a safe distance away before firing.
Personal opinion of Terror Stars as a weapon
Terror Stars can be a extremely devastating weapon when used in a medium to small sized map but you will find that in the larger maps the movement restriction is just ridiculous and you don't want to wait 60 weeks for your terror star to get where it needs to go.