Absolutely love this key generation method designed (?) by Blockstream 
Discussion
What is with the “valid final word”
That would be the checksum. You can't throw together any 12 or 24 words you want. The final word must be computed from the rest.
It's a very complicated thing to do by hand and you don't want to trust an internet connected computer. This is why airgapped tools that can compute the checksum like nostr:npub17tyke9lkgxd98ruyeul6wt3pj3s9uxzgp9hxu5tsenjmweue6sqq4y3mgl are so valuable.