To tack on ribbon, you can sew a few stitches at each break between the initials, loop the ribbon on alternating initials, or tie the thread off at each break.
For a neat finish, turn each end of the ribbon under and stitch ends.
Many of us remember the name tags or personalized, monogrammed ribbons sewn on the insides of our clothes when we were children to distinguish one child's sweater from another's. But personalized ribbon hand-stitched to the outside of a baby's sweater transposes whimsy over practicality. Use personalized ribbon to adorn other baby items such as a T-shirt or hat.