Complex sentence: Ramakrishna found Narendra a "Dhyana-siddha" (expert in meditation) and as an apprentice of Ramakrishna, he took meditation lessons from him, which made his expertise on meditation more firm.
Split into simpler sentences: Ramakrishna found Narendra a "Dhyana-siddha" (expert in meditation). Narendra as an apprentice of Ramakrishna, took meditation lessons from him, which made his expertise on meditation more firm.