UserBuilder.cs

public class UserBuilder
{
    private readonly string _firstName;
    private readonly string _lastName;

    public UserBuilder()
        : this(Guid.NewGuid().ToString(), Guid.NewGuid().ToString())
    {
            
    }

    private UserBuilder(string firstName, string lastName)
    {
        _lastName = lastName;
        _firstName = firstName;
    }

    public UserBuilder WithFirstName(string firstName)
    {
        return new UserBuilder(firstName, _lastName);
    }

    public UserBuilder WithLastName(string lastName)
    {
        return new UserBuilder(_firstName, lastName);
    }

    public User Build()
    {
        return new User(_firstName, _lastName);
    }
}

UserTests.cs

[Fact]
public void Some_test()
{
    // Arrange
    User user = new UserBuilder()
        .WithFirstName("Some first name")
        .Build();

    // Act

    // Assert
}