I'm trying to implement an SPV and I'd like to test it on a testnet full node but can't really understand how to do it from what I've found.
Download the Windows installer for Bitcoin Core from https://bitcoincore.org/en/download/. Install Bitcoin Core. Once it has installed, you should see a shortcut in your start menu with the name
Bitcoin Core (testnet). Run that. You now have a testnet full node.