I would start by learning the documented way to do this. Once you understand what is going on, you can tailor it to your requirements.
HOWTO: Routing all client traffic (including web-traffic) through the VPN
Using a VPS as the server, you will need a different iptables rule on the server:
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -j SNAT --to-source 12.34.56.78 # <-- Use your OpenVPN server's real external IP here
