I recommend using a small tub that either sits on the counter or inside a bigger tub. Fill the tub up with lukewarm water. Depending on the age of the baby, they may be able to hold their head up themselves, but if not, you can add a supportive sling in the tub for them to rest on. Tilt the baby's head back gently and use a cup to wet their hair, then wash their hair with a gentle, non-toxic shampoo. When you're finished, tilt your baby's head back and use the cup of water to wash the soap back out. That will keep the soap from getting into their eyes.