You can now use the npm package sequelize-auto-migrations to automatically generate a migrations file. https://www.npmjs.com/package/sequelize-auto-migrations
Using sequelize-cli, initialize your project with
sequelize init
Create your models and put them in your models folder.
Install sequelize-auto-migrations:
npm install sequelize-auto-migrations
Create an initial migration file with
node ./node_modules/sequelize-auto-migrations/bin/makemigration --name <initial_migration_name>
Run your migration:
node ./node_modules/sequelize-auto-migrations/bin/runmigration
You can also automatically generate your models from an existing database, but that is beyond the scope of the question.