Stepmom wanting to teach stepdaughters how to bathe properly?

My fiance and I have primary custody of his 4 daughters (ages: 6,7,7,10). We are both concerned they are not bathing properly (washing in the correct places) and feel like they may need a refresher course as their mom is not around and when she is, will not have that converstaion with them. There are no other female family members with in 100 miles so I (soon to be stepmom) am first on the list to teach this valuable life lesson. I am perfectly fine/comfortable with it but am having some trouble figuring out how to go about in a way they feel comfortable but will benefit from the lesson. Any suggestions?