Bip39-Standalone Or for quite higher security, you could "roll your own private" seed phrase with dice or maybe a calculator as offline procedures. Any individual else who discovers the phrase can steal the bitcoins. It need to be saved mystery, for instance it should not be typed into any Web-site. https://myeasybookmarks.com/story4072310/bip39-node-js-5-easy-facts-about-seed-phrase-generator-described