This is my first post but I have been doing 5:2 since January, losing 10.5 kilos.
My tip for newbies is to record everything you eat for a while until you get a good understanding of how many calories are in the items you eat. After a while this will become second nature.
I use my phone to track this (MyfitnessPal app). Its very easy and you can scan barcodes too.
IMO, this is not magic, just a technique that works for many that will enable us to eat fewer calories than we burn!. In my case I need to have a weekly calorie deficit of at least 1500 to lose weight.