What does it take to do private label supplements well from an efficacy and safety standpoint?

From a supplier perspective: Employ GMP-Certified manufacturing labs with a strong industry reputation for quality control –  that use pharmaceutical-grade ingredients and conduct third-party testing on ingredients/final products. 

From a provider perspective: to recommend supplements within guidelines backed by clinical studies, to cross-check any ingredient interactions with current medications, and to educate patients on potential side-effects.