Doing the strangest thing, loving the alien
Want to scan a Thargoid, but don't want to end up dead? Here's how...
This only applies to players with Horizons.
Lock and load...
First you need to purchase a Xeno scanner from station outfitting and ESSENTIALLY purchase a heat-sink launcher and fit them to your ship. If you cannot find these in the station you are at, use the EDDB.io website to locate a station near you that has them in stock (put "xeno scanner" in the "Station sells" box and then put your current location in the "Reference system" box). You are then set for a close encounter of the non-fatal kind. You'll be making a long trip, so consider fitting a fuel scoop if you don't already have one.
Remember to assign the Heat Sink launcher and Xeno Scanner to a fire group.
Finding Thargoids
Currently in the game the best place to find Thargoids is in the Pleiades sector, which means a trip of 150-200ly from the main bubble (depending where you start). Look on the Galaxy Map for the damaged stations and stations under repair. These systems have the Thargoids in their vicinity. Plot a route to one of these systems and get jumping.
I've arrived! What now?
Fly around the system in super-cruise and look for Unknown Signal Sources. You want to find a Non-Human Signal Source - Threat level 5 is a Cyclops, 6 is a Basilisk and 7 is the Medusa. If a Thargoid is there when you drop into the USS, it will approach to scan you.
At that point as soon as the Thargoid is within 500m, fire the heat-sink to make your ship go cold - this will stop the Thargoid from detecting your heat signature and it will not then consider you a threat.
Waste no time using the Xeno Scanner to get that scan! The scan only works within 500m.
If you don't go cold, the Thargoid will attack you and almost certainly kill you.
Once the scan is complete, back off and retreat to at least 1km away. As long as you don't get close or fire weapons, you can remain and observe the Thargoid. Usually they will wander around scanning wreckage and picking up components before warping away.
Live to scan another day
Once you've scanned or once the ship has left, jump out and head to the nearest station. You did it!