In general, offline storage involves creating private keys and then keeping the private keys on an offline device or paper. The easiest and safest method is using a seed phrase that will generate all your private keys. Hopefully, the official wallet software for the altcoin will support seed phrase creation, otherwise you may have to rely on third-party wallet software.

Without a seed phrase, the private keys need to be directly saved. For example, for STEEM your password (that begins with "P5") is the private key that should be kept offline.

This article may be helpful. It's about storing bitcoin, but it applies to all cryptocurrencies: https://en.bitcoin.it/wiki/Storing_bitcoins