note, you can use any DNS servers you like, the two above are OpenDNS's public DNS servers.

Edit /etc/pptpd.conf
At the end of the file "/etc/pptpd.conf", add:
localip 10.99.99.99
remoteip 10.99.99.100-199

These values do not have to correspond to your network. It is best to pick un-accessible/unused addresses here if you only want to use the VPN for Internet access.

Edit /etc/ppp/chap-secrets
The format for "/etc/ppp/chap-secrets" is [Username] [Service] [Password] [Allowed IP Address]
Add something like this to the end (replacing sampleusername and samplepassword with whatever you want):
sampleusername pptpd samplepassword *

Reboot pptpd
Finally, you can reboot the pptpd server with:
sudo /etc/init.d/pptpd restart

Edit /etc/sysctl.conf
Un-comment the following line in "/etc/sysctl.conf":
net.ipv4.ip_forward=1

The following command reloads the configuration (you can also just reboot at the end of this guide):
sudo sysctl -p

Edit /etc/default/ufw
Edit "/etc/default/ufw" and change the option "DEFAULT_FORWARD_POLICY" from "DROP" to "ACCEPT"

Edit /etc/ufw/before.rules
Add the following either at the beginning of "/etc/ufw/before.rules" or just before the *filter rules (recommended):
# NAT table rules