Priotizing VoIP over Data using QOS

Hi! We’re setting up a new office for our client, all physical were laid-out and some configs were done. Before we add a configuration to voice and qos, I would like to ask what could be the best way, practice and/or configurations in prioritizing voip over data?

VLANS: voice, data, wifi
VPN: OpenVPN (Server->Multi-client)

Thanks!