Here's what I do for my 9 m.o. dear daughter,
1st - sit her in a small circular basin with a bit of water to shampoo and out bath soap, then use a wet towel to wipe/wash of the suds,
2nd, transfer her to a long bathtub filled with water to wash her cleaner.
3rd, use a towel to dry off
Evening time, just wipe with wet towel.
After she did a big business, wash her butt in circular basin.