We introduced a cup when solids were added to our infants diet. We modeled using a cup at the end of his meals then allowed an opportunity for him to try. Through this repeated experience, he quickly became capable of successfully lifting the cup to his mouth independently to take a sip. This video was taken at 6 1/2 months.
I continue to be amazed at the abilities of children if given the opportunity. Just a little bit of time and patience, two things I can happily afford.