You will keep doing the raglan shapings just the way you were doing them, but you will BO all the neck part on the two sides of the 12 stitches you placed on the holder for the center. Kind of like this \__/. The ___ is the ones on the holder and you will BO on both sides of that. On one side the BO's will be on the knit side and on the other they will be on the purl side. They will be at the beginning of the row on the neck side.