For a shorter and more concise guide that also deals with IPv6 see this:
https://github.com/tool-maker/VPN_just_for_torrents/wiki/Running-OpenVPN-on-Windows-without-VPN-as-Default-Gateway
A change made by AirVPN in June 2018 during their move to "Gen2" servers may make this approach unworkable for people using uTorrent or other clients where you have to provide the IP address to bind to.
See this:
https://airvpn.org/topic/28494-tunnel-private-subnet-c