Sit upright n pour water from cup. I will pour one side first n cover ears then other side. If cranky day I will use cloth to sweep over hair repeatedly cuz of water running down face will irritate a cranky toddler!!! I started to get my kid used to water running down face since 6 mths old tho. Bit by bit everyday. now my kid is 21 mths.
I play with my daughter when in the shower, so instead of just asking her to look up (which she understands but not necessarily listens), I ask her to spot my face features, or the shapes in the bathroom which she has to look up at, while washing using a showerhead/scoop of water so it runs down backwards.