By using "do", you suggest the action(s) for your hair or nails (ie. combing, colouring, for hair; painting, paring for nails). For another example, "do the report" means research and write the report. How else could the report be completed? ;)

Do your hair = this refers to what ladies do to make their hair pretty, hair stylists do at the beauty salon. Do your nails is a manicure ( and pedicure too) it refers to clipping the nails and/or painitng them. Hope this helps John

Do your hair Means to actually make your hair look nice Do your nails To clean, clip, apply nail varnish and so on.