。
The pharmacist compounded it according to the prescription.
Literal
Pharmacist [topic-は] prescription according-to compound-do-[favor-てくれた].
処方どおりに means 'exactly as the prescription specifies' — the suffix ~どおり attaches to a noun to mean 'in accordance with.' The auxiliary ~てくれる marks the pharmacist's action as a favor done for the speaker; dropping it would be grammatical but would lose the sense of gratitude. 調合する is the specific verb for a pharmacist's act of mixing a compound medication — more precise than generic 混ぜる.