Make a taller body for the right head than for the left head, since this is the older sister/friend of the pair.
Don’t detail the hands or feet too much at this step (same as the faces).
Make big eyes (following the horizontal guideline) and eyebrows above them, with a small triangular shape for the nose and a horizontal curved line for the mouth. Give her a bob hairstyle with bangs.
If you want your girls to be sisters or very close friends, you can actually give them more similar hair/facial expressions–it’s all about the choices you make for your drawing.
Draw a sleeveless dress for the bigger girl and a dress with short sleeves for the smaller girl. Add collars for both of them, but draw them in different styles. Make crinkles on both dresses. Finish up with the shoes.