
Bring up the Workshop menu and a ‘Special' category will appear. The Minutemen's settlement at Sanctuary Hills has number of such lots. Whether you decide to build the teleporter with a faction or not, you first have to start with a large empty lot in a Settlement. When you express your need to have a Signal Interceptor built, the faction leader will tell you to talk to the faction's tinkerer or scientist. Whichever faction you choose, you'll need to complete the first two quests pertaining to each faction, one of which being the quest where you encounter the faction.

That would be Paladin Danse from The Brotherhood of Steel, Preston Garvey from The Minutemen, and Desdemona from the Railroad. To get help from a faction, talk to the leader of your faction of choice. The benefit of getting help from the respective scientist of any of the three factions is that they'll give you advice on how to put the teleporter together.

To build the Signal Interceptor, you have four options:

Virgil will give you the plans to build a Signal Interceptor, which will allow you to teleport to The Institute and rescue Shaun. After Tom takes a look at the chip, fast travel to Virgil's lab in the Glowing Sea.
