Assassin's Creed Valhalla

Refer to Spelunker (10G) for accessing and starting this DLC, as well as completing the first tomb. After completing it, you'll be shown (vaguely, on a rock wall) the locations of the three other tombs. Of those three other tombs, you only need to complete two of them for this achievement (since the introductory tomb counted as one of three for this achievement).
 
IMPORTANT NOTE: As noted in the roadmap, each of these three tombs is marked as completed (towards this achievement) as soon as you loot the gear chest(s) at the end. However, each of these three tombs also contains a tomb fragment located at the end. These tomb fragments are not needed for either of the two achievements for this DLC, but there is a high likelihood they will be needed for future content. It is therefore highly advisable to pick them up before you fast travel to where the next tomb is, so you don't need to backtrack for future content.
 
For a guide of all three tombs, where they are and how to complete the puzzles in them, refer to THIS LINK.
 
The only problem I had with that guide was that it didn't show the actual entrances to the tombs, which is hard to find in most cases. In particular, Boudicca Tomb in East Anglia and Venutius Tomb in Eurvicscire. To help, here are screenshots of each.
 
Boudicca Tomb Venutius Tomb