In the same namespace (or folder) that you created your form in Step 1, create a folder called templates. For example path/to/my/app/forms/templates.
This is where you will build your custom template packages. For example path/to/my/app/forms/templates/flat where flat is the name of your package.
You can also override an existing template by simply copying the package folder from the vendor directory. For example, copy vendor/helmut/forms/src/templates/bootstrap to path/to/my/app/forms/templates/bootstrap and then make your desired modifications.
Template files can be rendered by specifying the engine in the filename extension. For example path/to/my/app/forms/templates/flat/form.mustache.php will use the Mustache engine. Twig and Blade are also available.
So that fields can be simply dropped in, each one has it's own templates directory. See creating fields.