I didn't start expressed milk with any of mine so early on so not sure of any great advice myself but just wanted to point you in the direction of the breast and bottle feeding board where there are lots of people with some good breastfeeding advice around.
Hi i'd leave it a little longer before you start this. guidance is when bf is 'established' - whenever that is but i was told not before 6 weeks kelleymom has good advice on quantities, but dont get too hung up on exact amounts, just give baby what they will take and you will discover this by trial and error. i would say from experience express about twice to feed once in the early stages of expressing as you will never manage to express as much as baby can take in one sitting - your LO is very efficient at extracting milk from you, also depends on how good the pump is that you are using. i started off with hospital pump (SCBU baby) and 6 months later am now back at work using a manual pump 4 times during the day (avent ISIS) to produce the milk for 3 feeds the following day. hope this helps a bit good luck!