I recently bought a VPS to create a VPN application, but it looks like there isn't any tutorial anywhere about it.
I saw the ics-OpenVPN project on GitHub, but I don't have any idea how to use it!
How can I connect to a Linux server from my phone?
First of all, install OpenVPN on your VPS server. Although I do not have experience to install OpenVPN on VPS server, I think you can follow this tutorial it looks easy How to install OpenVPN on a Linux VPS.
After completing your installation, you will get the .ovpn configuration file. Now to connect with your VPN server from your Android app, you need an Android client software like the ics-OpenVPN Android client.
I have also developed an Android VPN app based on the OpenVPN library, and you can use my source code. It is open source. The instruction of changing .ovpn file is given in the GitHub repository.
GitHub: Cake-VPN