I did it with DS. Had to start topping up at 3 months due to weight loss and weak suck. The stress of that meant I couldn't express very much and my stash quickly got used up.

My aim was to get rid of the comp feeds which we did at around 9 months and continued breastfeeding until 14.5 months.

I did:

Express after each feed
Top up with EBM or formula after each feed using a supply line. This meant he was still at the breast stimulating my supply. I used a supply line for about 3 months until his suck improved.
Always gave breast first, then bottle. Every 4-5 days reduce the amount of formula by about 10mls. If he still wanted more, I gave him breast again. This took ages and we sometimes went backwards. But we got there in the end.

HTH