It would be best to make a cold storage for a wallet. Which is essentially in laymen, storing the wallet offline.
One methodology of offline storage would be creating a paper wallet.
An image that can be printed onto paper with a Bitcoin or Ethereum wallet consisting of 2 ‘keys’.
The public key (your wallet address / how other people send currency to you), and the private key (how you send currency).
These paper wallets can be physically printed or saved as a file. Which on a USB is where I would recommend.
The first thing you need is a USB drive, most likely you'd format it.
Next would be to generate the wallet, this can be done either online or offline, you have increased security offline so that's the method I would recommend. As you're asking about Ethereum, you can generate a Ether wallet using MyEtherWallet.com or downloading from GitHub [https://github.com/kvhnuke/etherwallet/releases/tag/v184.108.40.206] and running MyEtherWallet offline by opening index.html file in any web browser.
Once you have generated the address you would select 'Print' > Open .PDF in preview > Then save directly to SSD/USB Device.
Next would be to deposit the desired currency to the wallet. (Ensure you're sending the same currency to the wallet its designed for, DONT send btc to an Ether wallet.)
After which you would confirm the transaction via one of the desired (https://etherscan.io/),(https://www.etherchain.org/), or (https://ethplorer.io/), theres many others those are just the most popular.
The full amount of the wallet’s value should always be withdrawn because once the wallet is scanned, the private key has been exposed. This is a cumbersome process which is why cold storage is mainly intended for savings, not daily spending. For spending, or wallets connected to the internet like accounts with Coinbase are suitable.