Bip39 Checksum. Entropy values have to be sourced from a solid supply of randomness. What this means is flipping a good coin, rolling a fair dice, noise measurements etc. Don't use phrases from books, lyrics from tunes, your birthday or steet tackle, keyboard mashing, or anything you think is random, https://links2directory.com/listings13033218/bip39-mnemonic-generator-https-linktr-ee-bip39-details-fiction-and-bip39-word-list-pdf