Hello, I have been exclusively breast feeding m...

Hello, I have been exclusively breast feeding my DS since birth. He is 8 weeks old now and I'm ready to have a night out with some friends and leave my DS home with hubby. I have been using an electric pump to express about 20ml a day and freezing for emergencies. How do I express for say one bottle a day? When is the best time to express the feed? Do I express the full amount in one go or add multiple expressed volumes together? Any advice would be appreciated.