I have long-ish, straight hair.
I don't wash my hair in the shower, mainly because it tangles and I don't want to lean backward, afraid of falling.
So, I wash my hair in a medium rectangle kitchen dishpan.
I lean over the sink, shampoo. Let the shampoo sit in my hair by wrapping a flour sack towel around my head.
Then, ten or more minutes later, I keep rinsing out the shampoo.
Then, I add a capful of apple cider vinegar into the tub of clear water, which de-tangles my hair and makes it very smooth and clean.
Keep in mind that my faucet spout is tall and rinses out my hair / head quite well. I move my head from side to side in order to rinse all of the shampoo out from my temples and back of my head.
Hope this suggestion helps someone.
'More or less', 'Right or wrong', 'In general', and 'Just thinking out loud ' (as usual).