I would start introducing now - best for both baby and a gradual reduction of your supply. Is she on any kind of routine?
I started by doing the evening bottle (just before bed, about 6pm). On the tin it will state what is the recommended amount for a baby of a particular weight or age. Start with stage 1 formula, suitable from birth.
I gave the evening bottle for about 2 weeks, still BF at about 10.30pm (just before I went to bed, and also during the night if he woke). Then I began giving him a bottle around lunchtime (11am). Did that for a couple of weeks. Next was the about 2.30pm feed. Then LO started sleeping through, but I was still BF at 10.30pm - I did what's called a 'dream-feed' - feeding the baby while they're still asleep. Just pick them up, sit down in their room with the lights dim, no talking and feed for about 30 minutes.
I kept just the morning feed for a long time, maybe a month? - So he was getting BF in the morning and last thing at night, and 3 bottles during the day. Once he was sleeping regulary through from 10.30pm till 7am, I dropped the dreamfeed, and he slept a whole 12 hours!!
You could keep the morning and dream-feed for ages if you like, it might work with your working day?
If I felt engorged with milk, I'd express just enough to feel comfortable (about 1-2oz) and mix it with his formula.