Make your formula up as you normally would, just less of it, and then top it up to the full amount with cow's milk, and then warm it up (babies generally prefer it warmer).
i.e If your baby has 8oz bottles;
Maybe start with 6oz formula, 2oz cow's milk,
Then go to 5oz formula, 3oz cow's milk,
Then 4oz formula, 4oz cow's milk..
Just gradually getting more cow's milk and less formula. You might need to do a few days at a time, or you might be able to jump up a step (like adding more than 1 more oz of cow's milk at a time).