ITN was the incentivised test net,which has already ended. For the testnet you had to create a rewards wallet with 15 seed words. If you don’t have those as far as I know there’s no way to redeem the rewards Fr the testnet.
However we can now stake on the mainnet. You said you have your 24 word shelley wallet, so you can use Yoroi l, Daedalus or adalite to stake your mainnet Ada. On deadelus you don’t have to withdraw the rewards, when you send Ada it will also take it out of the rewards account (there is no separate 15 word seed for that, it’s all connected to your 24 seed phrase). On yoroi there is a link on the dashboard to withdraw rewards to your “normal” wallet, again no extra seed needed. When you use the withdraw option you get prompted if you also want to revoke your staking certificated (this is only needed when you want to completely undelegate and get the 2 Ada deposit back, so normally this is not needed).
If you use Daedalus make sure to go to daedalus wallet.io or use the link on the cardano.org homepage to download the recent version. For yoroi you download it via the chrome or Firefox extension library.