DigitalOcean has a good article on setting SoftEther VPN on one of their droplets.
The instructions worked for me without just fine, with a couple exceptions:
1. When you attempt to use the command line vpncmd tool to set up the server, you may have to specify localhost:5555 rather than using the defaults.
2. You may have to create a group for the test user before creating them and assigning them to a test group. This can be done with the command `GroupCreate test`.
3. If you are using a firewall, you will need to open up the ports used by SoftEther. To figure out what ports it’s using, do `sudo netstat -atulpn | grep vpnserver`. By default, SoftEther will listen on TCP ports 443, 992, and 5555. If you’re using L2TP/IPsec, make sure UDP ports 500 and 4500 are open as well. If you’re using ufw for your firewall, you can see which ports are open/blocked with `sudo ufw status verbose`. To get an idea of which ports each VPN protocol you’re using requires, check out the SoftEther specifications.