I use an old smartphone to generate air-gapped wallets offline

TL;DR: I use an offline old smartphone with Metamask to create secure air-gapped Ethereum wallets. Generate a wallet, note the seed phrase, validate it, clear data, rinse and repeat. Never connect the phone to the internet.

Steps:

1. Update phone firmware (optional).
2. Download Metamask or preferred wallet app.
3. Disconnect from all known Wi-Fi networks, turn off Wi-Fi and Bluetooth, and remove the SIM card.
4. Open Metamask, generate a new wallet offline.
5. Note down the 12-word seed phrase and public address (0x***).
6. Clear all Metamask data in settings.
7. Import the previously generated wallet to also validate the seed phrase you just noted down.
8. Copy the public address for future fund transfers.
9. Clear all Metamask data again.
10. Repeat steps 4-9 to generate additional wallets.

The phone must never connect to the internet ever again. It should solely serve the purpose of generating wallets.

When the time comes to move your funds, use the seed phrase to create a new wallet on a malware-free PC or laptop. For added security, consider obtaining a dedicated inexpensive laptop solely for this purpose.