Slowly. I recommend starting solid foods at 4 months for bottle fed babies and 6 months for breast fed babies. I recommend starting one new food at a time with small amounts. I increase slowly over 3 to 4 days to observe for any reaction then start the next food. I usually start with cereal mixed with breast milk or formula fed with a spoon. Next I offer yellow and green vegetables then fruit.
Feed food, then milk. At the beginning of feeding time, when the baby is hungry, give a little baby cereal or baby food. The baby doesn't have to eat much solid food at first (he can just enjoy the new experience of the textures and flavors). Then feed the breastmilk or formula to finish that meal.