Bip39 Library. Theoretically, one can Imagine out the mnemonic phrase by his personal, but It's not at all Risk-free, since a human can't do an awesome occupation with the random generation. The best way to protected 1’s wallet will be to take a mnemonic phrase produced by the program code. https://deanzzzzz.gigswiki.com/5278036/the_anatomy_of_a_bip39_phrase_entropy_phrases_and_security