Girls, how do you correctly switch to mixed feeding?

Girls, how do you correctly switch to mixed feeding? The husband bought Gerber Gold and gave the son he doesn't drink today, I tried too, he doesn't want at all. A bottle of 6+ from Avent. We rarely gave formula in the first months when we went for a walk, the bottle was also from Avent, but the baby ate NAN. His face does this "??".