Of the text and video guides I found, these were the two most useful. I will say that I think the video wins out as the author(s) give a bit more detail so you don't accidentally miss some. Something worthy of note is that you don't have to finish the mission to get credit. So if you've completed the game, once you have the dataslates for the level you can head back to your ship and onto the next mission.
Text:
https://www.thegamer.com/warhammer-40000-space-marine-2-data...Video [Note timestamps in description]: