Peel and dice the sweet potato and the carrots. Slice the leeks (white and pale green parts only). Saute the leeks in a little olive oil or vegetable oil for a minute or two. Then add carrots, saute for another 3 minutes. Add the sweet potatoes and cover it all with chicken broth (or water) and simmer until soft, about 20 minutes.
Blend until you have the consistency you want depending on the age of your baby.
Use any canned or homemade chicken broth. If avoiding chicken, use water.