As other said, it’s up to you, it blend well with the rest of the base game (tho it have a different ambience and thematic) so you can install both and treat it as a whole, or play the base game first and then DLC. An important note is that some part of the ending of the DLC are not shown if you haven’t discovered some part of the base game (can’t say more to avoid spoiler), so if you decide to play DLC after the base game do it on the same save, do not start on a fresh one.

At one point you will know what to do to reach the end of the base game, I personaly think it is at that moment that you’ll have to choose between completing the DLC or reaching the ending. Until that, focus on discovering the base game.

tldr: both base game -> DLC -> ending or base game -> ending -> DLC are fine, it’s up to you.