Sort posts alphabetically by custom field

I’m creating a listing of people using custom fields to enter first name and last name…

I want to display a list of those posts displaying the ‘Last Name’ custom fields first alphabetically.. i.e;

• Ally, Oop
• Smith, Jones
• Tegan, Bob
• Uncle, The Man From

Someone help me pleeeease? I’m about ready to scratch grooves in my face and stab my monitor with a fork.