Yes the clues are subtle and it is not explained exactly what to do (in the middle between a good and a bad design choice) perhaps I could add some dialogue hint
spoilers
the drawings on the necronomicon are important but as you can see from the instruction sheet the drawings are not in the right order. What allows you to perform the first ritual (there are 6 bodies) is to remove the right organ (see second page of the necronomicon) and insert the right object (see first page of the necronomicon)
