Use a live Linux on a USB disk without network access and install Electrum on it with password protection.

1. Live Ubuntu on USB https://ubuntu.com/tutorials/create-a-usb-stick-on-windows#1-overview
(You can do a full install too but it harder)
2. Download and verify Electrum
3. Unplug your modem, disconnect internet, Lan cable, etc.
4. Boot into the USB with the live Linux
5. Install Electrum
6. Generate a new wallet and write down the seed phrase
7. Send your funds to this cold storage.

Note: you can and should test this setup first and see if you can create it correctly and use it correctly too. You can test using testnet with some free coins and/or with a small amount of bitcoin in a small tx.
To spend from this you can to repeat most of the above steps specially if you didn't enable persistence on the disk. You can read more about cold storage on Electrum docs.
There is an old guide with more details here: https://bitcointalk.org/index.php?topic=853288.0