FWIW, I started expressing around 2 weeks (I think it was at 10 days, in fact) -
There is SO much conflicting advice about expressing and bottle feeding EBM. I'll tell you what my MW said, cos it worked for us, and I genuinely believe DS being able to do a feed of expressed milk was a lifesaver for me. To go from never getting more than 2 hours sleep to getting 3, or even 4! hours in one go...Was a huge help.
DS was feeding well, in that he was gaining weight and had good latch, and I had no pain etc - althouth he was a very frequent feeder, and an epic cluster feeder, so it was v tiring for me.
MW said introducing a bottle feed once a day, or every 2 days, was very unlikely to cause nipple confusion if DS wasn't having latch problems to start with. She said don't go mad, and pump for hours in one go, but to just express a little at the start and end of each feed. That way you can build up a little daily 'stash' without sending your supply rocketing, and getting engorgement problems.
Get DS to offer the bottle when you aren't in the room. Wait until baby's hungry, but not screaming. If your DD just has a bit of a play and gets a little milk down to start with, that's fine.
(interestingly, my MW said that there's a small window of time to introduce a bottle, which is BEFORE 6 weeks. After that, babies find it much harder to learn, and many refuse to take a bottle at all. There are lots of posts on this forum from women desperate for their older baby to take a bottle, as they are returning to work, and want to express - they were told to 'wait', and now their DCs won't take it at all)
If a baby doesn't take the bottle at first, don't get stressed and disappointed, they might be fine next day, so stay calm and try again.
What I did was express in small amounts during the day - mainly after feeds, from the same boob DS had been feeding from, as I never seemed to have time to do it before, DS always seemed to be asking for a feed...
Then after feeding late at night (basically when DS had temporarily zonked out at the end of a marathon cluster feed), I would stagger off to bed, leaving DP up with DS.
DP would offer bottle of expressed milk on demand, same as I did with the boob - so basically I got woken when the milk was gone and DS was peckish again.
I found having 'skipped' a feed, my boobs were quite full, so I had to express a bit when I woke, or after I'd fed DS from one side, to get comfy again. I couldn't really go more than 4/5 hours max without getting engorged, but 4/5 hours was the absolute maximum break I got anyway.
I know expressing doesn't work for everyone, but my DS never had confusion, I found expressing easy and fast (with a hand pump), and DS still takes a bottle a night now, or if he's out and about with DP. It bought me a break and sleep when I desperately needed it.