Call of Cthulhu uses percentile skills: roll d100 and succeed if you roll at or under your skill. Difficulty often shifts the threshold (Regular/Hard/Extreme) and Sanity loss is a core pressure loop.
At-the-table steps
Pick the relevant skill (e.g., Spot Hidden, Library Use).
Roll d100 and compare to your skill rating (and difficulty threshold).
Apply consequences: information gained, complications, or Sanity loss when appropriate.
Example: Spot Hidden 55%: a roll of 42 succeeds; a harder threshold would require a lower roll.