You can start with studying research work about Chinese firewall to get the idea how DPI usually works [1]. Then you can start up a Wireshark and try sending different packets and see which are blocked and which pass through, or experiment with modifying VPN packets to make them pass through.

To experiment with this you need to buy a VPS abroad.

If you don't want to do that then you can search for existing utilities like: GoodbyeDPI, XRay/reality, AmneziaVPN etc.

[1] https://gfw.report/publications/usenixsecurity23/en/