Download the latest walletgenerator.net here: https://github.com/MichaelMure/WalletGenerator.net
Move zip to an airgapped computer & unzip it.
Doubleclick index.html.
Generate wallet with a strong password.
Save the password on the computer, using an additional layer of encryption. (or just never ever ever forget it. or write it down and save that somewhere offline super super safe.)
Save the Keystore file (encrypted JSON). Store it on this airgapped computer that will never access the internet ever.
Save your address as well. This is not secret. You will need to address to send ETH to yourself later.
Copy the encrypted private keys and password and address to a USB drive. For extra security: encrypt said USB drive. Win and Mac instructions.
Write down the encrypted private key and password on separate pieces of paper. Don't print them because all the printers have insecure wifi these days and store shit and we're being ultra secure here. Triple fucking check that you have written them down correctly. Store these papers in physically separate locations than the USB drives and airgapped computer.
Go to the view wallet details tab and type in your private key / password from your piece of paper to ensure they are correct and access your wallet. Check that the address you wrote down is the same.
Refresh that page. And copy/paste your private key or JSON and password from your computer or USB drive to ensure they are correct and can access your wallet. Check that the address you saved on your computer or USB is the same.