Any feeding techniqe?

my six months old baby drinks only 15 oz -20 of formula a day. her doctor told me to give her total six table spoons of rice ceral aday.3 table spoon at morning and 3 at or day night. gerber food is not important becoz her milk intake is very little. problem is after taking 2/3 spoons she dose'nt like that. I tried her to keep happy, like i turn on music , I play piano. she just keep lock ing her mouth. she is also drooling alot. any technique to feed her? also how can i increase her milk ?