1. Head over to Supabase and log in to your account.
2. Click on the "New Project" button to start a new project.
3. Set up your project settings, like the database password and region.
4. Once the project is created, go to the Supabase dashboard.

1. In the Supabase dashboard, find the "Table Editor".
2. Hit the "New Table" button to make a new table.
3. Name your table and set up the columns as needed.
4. Save the table schema.

1. In the Supabase dashboard, go to the "SQL Editor".
2. Write a function for custom errors. For example:

CREATE OR REPLACE FUNCTION validate_user_registration()
RETURNS trigger AS $$
BEGIN
    IF NEW.email IS NULL THEN
        RAISE EXCEPTION 'Custom Error: Email cannot be null!';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

1. Run the script to create the function.

1. In the same "SQL Editor", create a trigger to use the function you just made. For example:

CREATE TRIGGER check_user_registration
BEFORE INSERT ON users
FOR EACH ROW
EXECUTE FUNCTION validate_user_registration();

1. Run this script too.

1. In your app, use the Supabase client to talk to the database.
2. Handle the custom error messages in your front-end code. For example, using JavaScript:

const { data, error } = await supabase
  .from('users')
  .insert([{ email: null }]);

if (error) {
  console.error(error.message);
  // Display custom error message to the user
}

1. Try to insert a record that breaks the custom error rules.
2. Check out the custom error message that pops up.
3. Make sure the error is handled correctly in your app.

This way, custom error messages fit right into your Supabase project.