You will need an SRV to pop this.
The wiki entry lists locations for a number of different Guardian ancient ruins:
http://elite-dangerous.wikia.com/wiki/Ancient_RuinsSynuefe XR-H d11-102 - planet 1B
IC 2391 Sector GW-V b2-4 - planet B 1
IC 2391 Sector ZE-A d10 - planet C 3
Synuefe XO-P c22-17 - planet C 1
Synuefe ZL-J d10-119 - planet 9 B
Col 173 Sector KY-Q d5-47 - planet 8 C
HIP 39768 - planet A 14 F
Synuefe TP-F b44-0 - planet CD 1
Col 173 Sector KY-Q d5-47 - planet 9 A
Synuefe NL-N c23-4 - planet B 3
Col 173 Sector KY-Q d5-47 9 A
If you're within 1,000 light seconds, you get an "Ancient Ruins" point of interest marker in the navigation menu.
Land nearby and deploy the SRV. Near the marker you will find the ruins themselves. You need to scan the cuboid structures, the ancient obelisks, to unlock the achievement. Select with Y then use LB to scan. You will get an "Ancient Civilisation Information Downloaded" message and the achievement will pop.
The first location in the list worked for me. I had visited HIP 39786 before that and found an odd bug where the ancient ruins marker spawned underground. I'd avoid that one.
Credit to Campo for initially pointing out that an SRV is required :)
EDIT 21/3/18: it looks as if the 3.0 update, which added new Guardians content, may have changed what needs to be scanned to unlock the achievement. See the comments below from DEEVIUS and Boba Fett - and in particular the very helpful post from DEEVIUS on 21/3/18 with links to pictures of what now needs to be scanned post-3.0.