If you just stop feeding or expressing then the supply and demand will drop and all will dry up. Be aware though that it can take quite a while. I still had some milk more than 6 months later - but not enough to feed with.
You can either stop all at once and just deal with any engorgement (you may not get any if your supply is now low anyway due to formula feeding) for a few days, by taking paracetamol for any pain and just wearing a good bra and plenty of breast pads to deal with leakage. The excess milk will just be reabsorbed and because your supply isn't being stimulated, then you will stop making milk.
Alternatively you can drop a feed at a time, with a gap of around 3 days or so between each dropped breast feed.