Supplementing Formula

So my milk supply is taking a huge hit with this pregnancy and I have done everything I can to get it back up without results. I'm starting to supplement formula and still want to nurse as much as possible. My son is 9 months. Do I nurse and then offer a few ounces of formula after? Or do I skip nursing and offer formula instead for that session? Do I need to combine breastmilk and formula if I nurse first and follow up with formula? Do I need to combine breastmilk and formula if I only offer formula for feeding for a while? What did you do if you did a combination of breast and bottle?