Apply after essence as the vitamin C step. Into The Gloss says Lily maybe uses a vitamin C serum from KORA Organics, and Harper's Bazaar later lists KORA Noni Bright Vitamin C Serum in her nighttime product lineup.
Noni Bright has since been replaced by KORA's current Kakadu Plum Vitamin C Serum, so this links the newer same-brand vitamin C.