If your goal is to separate your identity from your activity, then as a minimum:
  • Pay anonymously, preferably via LN rather than on-chain BTC.
  • Do not provide identifying information when signing up (register with a dedicated pseudonymous email account if needed).
  • Do not use this VPN in a way that leaks identifying information to the VPN provider, such as visiting your personal or employer's website.
What IP addresses you connect from may also let the VPN provider identify and track you. In particular, consider the implications of connecting from both your home/mobile network and e.g. your employer's network.
For better privacy you might access this VPN via another VPN provider. The VPN you connect to directly can track your IP address / physical location but not your online activity, while the remote VPN can track your online activity but is hopefully not able to identify you. Setting up such a system can be complicated, though.