mixed feeding

Hi, i need to mixed na ang breastfeeding and formula milk kay baby. Kaso ayw nyang .ag formula milk kahit na gutom na sya. Nag cacry lang sya and refused to take formula milk. Ang results, i still need to breastfeed my baby. Mahina nag supply ng breastmilk ko so whenever I need to go out nang matagal, wala akong naiiwan kaya kailangan na dn masanay si Baby sa formula. Any help, suggestions kung pano formula si baby?