If you select 12 words, there's a big probability that your seed phrase doesn't pass the checksum.
I intend to flip a coin 121 times, convert to BIP39 words and then enter them into my HW with a random 12th word until it accepts the mnemonic as valid (passing checksum). This way I'm not relying on the HW RNG.
Do this to simplify the process. It will give you the full 128 bits of entropy and should make finding the checksum easier:
1. After flipping 121 times to get the first 11 words, flip 7 more times to get a number between 0 and 127.
2. Multiply that number by 16.
3. Only one of the words that corresponds to that number or one of the 15 that follow will give you a valid seed phrase when used as the 12th word. Try each of them until you find one that works.
For example, flip 121 to get 11 words
raccoon weird maze affair stomach fall whisper direct unveil chase enhanceFlip 7 times to get the number 101. 101 x 16 is 1616. Trying each of the words corresponding to numbers 1616 - 1631, you will find that 1619 "
skill" works as the 12th word.
So, the phrase is
raccoon weird maze affair stomach fall whisper direct unveil chase enhance skill