How to connect the users table to the roles table (using the table user_roles)?

I need to create a basic database for user authentication. So far, every user has a name, a password and a role. I've found this somewhere on the Internet, which looks quite promising:

create table if not exists users (
  id int unsigned not null auto_increment,
  username varchar(100) not null,
  password binary(60) not null,
  primary key(id),
  unique(username)
);

create table if not exists roles (
  id int unsigned not null auto_increment,
  role varchar(100) not null,
  primary key(id),
  unique(role)
);

create table if not exists user_roles (
  user_id int unsigned not null,
  role_id int unsigned not null,
  unique(user_id, role_id),
  index(user_id)
);

But... if I would create new users, how would I fill the user_roles table? I have a feeling there is some "automatic way" to do this, but I have totally no idea (being a database noob :-)). How could I somehow connect a user to a role?