Tips on bathing baby in the sink
The safest way to bathe baby in the sink is using a baby tub. They have infant inserts to keep baby's head above the water, then when your child is big enough you can remove the insert and use the tub by itself. Charlie will be 5 months old next week and can still fit with plenty of room, but he enjoys bathtime a little too much and splashes around like crazy, so no more baby tub for us.
If you aren't going to use a baby tub, I suggest holding baby in a towel in the sink. The wet towel will give you a better grip, wet babies are sooo slippery!