I translated it myself, I don’t know if it is accurate, it is for reference only:

Studying knowledge and reading requires patience, (to) think carefully to understand and experience, and must not be careless . (It's like) counting objects with many layers, wrapped in the innermost (thing), with no chance to see. (That is) I must go to the (outermost) floor today and be able to see the (inner) floor. I went to the (outermost) floor tomorrow and could see the (inner) floor again. Only by removing its skin can you see its flesh. Only after removing its flesh can you see its bones. Only after removing its bones can you (finally) see its marrow.

Meaning: Studying knowledge and reading requires patience and not carelessness. To experience and understand slowly is like peeling off skin. Only when you peel it off can you see its core (the true connotation, which is the "marrow" mentioned by Zhu Xi above).