What dkbit98 was implying is that using open source wallet is better because the source code is available to the public. A good and reputed wallet of that category which can be recommended is Trezor. But, it is good to keep Trezor in a place it can not be stolen, or better use passphrase along with seed phrase so physical attacks can not reveal the keys generated by the Trezor wallet.

If you want to know how to generate a cold wallet using an online wallet, you can use Electrum which is open source but only support Bitcoin. You will need two devices for it, one as watch-only, and the other as the cold wallet which is used for signing transactions. You can follow below guild:

https://electrum.readthedocs.io/en/latest/coldstorage.html