Bip39 Protocol 1 example of manually generating entropy would be flipping a coin. But when an attacker offers you a coin that is weighted somewhat in favor of heads, your Original source of entropy should have been corrupted. Tip: Some people spotlight each 16th word inside of a printed wordlist https://getidealist.com/story20267256/bip39-generator-with-balance-considerations-to-know-about-seed-phrase-generator