Video: https://www.youtube.com/watch?v=jqJvL7cA8jE&ab_channel=Exakan

There are different perks available, read here about all DS perks:
https://psteamcommunity.yuanyoumao.com/workshop/filedetails/discussion/2719075597/3194742149936159522/

The basic ways:
Spectrals - can build one when using their crisis perk "Interstellar Overclocking". Its currently the strongest superweapon in DS.

Halo origin - you can build a Halo when finishing the special project.

Halo Array - from an ascension perk which is available for all empires.
Its called "Halo Array".

Darkmatter Ripper - from an ascension perk which is available for all empires.
Its called "Shattering the space".

All of them are placed like gateways - outside of the gravity of a star.
They have multiple stages and need a lot of resources to build and upkeep.

1.Idlemode
At some point the description of ANY superweapon will say "Idlemode"
<<not ready to fire>>.
Idlemode means the weapon is done, but needs to charge up. To do this, upgrade to the next stage.

2. Firemode
The megastructure description changes to "Fire mode" <<ready to fire>>.
This means the weapon is ready to fire, but you have to place a "Locator".

3a. Target selection
You can build the small megastructure "Locator", it costs only some energy and takes no time to build. You can build it in friendly, neutral or enemy systems.
Select the star to build it.

3b. Target selection - limits and rules
Stellaris has some limitations when placing a structure, so keep this in mind:
- the construction ship must have the fleet stace "Passive", NOT!! "Evasive", otherwise it will not work in enemy space
- if you target a friendly or neutral system and fire the weapon, they will declare total-war on you
- there is sometimes a bug where a system isnt a valid target, but this is Stellaris' fault and I cant fix it
- sometimes FTL-inhibitors on starbases block the placement

Question: Why do these problems appear?
Simple, because the construction ship needs a clear path to the target. If its blocked, it cant go there to build.

Question: It has to go there!?
No, because the structure is instant. But thats how Stellaris works and sadly I cant change it.

Read step 3a. Target selection and 3b. Target selection - limits and rules.
These limitations cant be removed because its how Stellaris works.

However, an easy solution is to push the frontline closer to the enemy. You dont need to be close, but sometimes the weird mechanics work better when your own space isnt so far away from the enemy empire.