Harvesting a creature's components requires two checks... an Assessment check and a Carving check.
If both checks are made by the same person, they are each made with Disadvantage.
Assessment check = 1d20 + Intelligence modifier + proficiency bonus (if proficient in the skill on the above table)
Carving check = 1d20 + Dexterity modifier* + proficiency bonus (if proficient in the skill on the above table)
*or Spellcasting ability modifier if it can be ritually carved.
For some creature types, magical rituals can be performed instead of getting elbow-deep in grisly viscera. When making a Carving check to harvest an aberration, celestial, elemental, fey, or fiend, a carving harvester with a spellcasting ability can make the Carving check using that ability instead of Dexterity.
For a spell or magical effect to have any influence on the outcome of harvesting, it must affect a harvester for the entire duration of the Harvesting check. For this reason, spells with a duration of 1 minute, like bless and guidance, never confer their bonus to the result of the check. A spell like enhance ability, which lasts 1 hour, could confer its advantage to a Harvesting check so long as the spell begins before the check starts and does not end until after the check is completed.
Helpers. Creatures not involved in assessment or carving can help! The number of creatures that can help depends on the size of the creature being harvested (see table below). If a helper has proficiency in the skill associated with the monster’s type, the helper adds its proficiency bonus to the Harvesting check’s result. If the helper doesn’t have this proficiency, it adds half its proficiency bonus rounded down, instead. Helpers must help for the entire duration of the harvesting procedure to add this bonus, and are considered assessing harvesters.