How do you prep a wall for paint that has flat paint on it?

Here in australia we have a product called sugar soap that is used to clean walls ready for painting. If it is sheet rock, I recently painted mine with no prep. Maybe just use a primer first or do a test patch.

tsp followed by clean water to rinse

Well, the key is to use a rag that's "damp" not dripping wet. But unless the wall is truly dirty--greasy, obviously dirty, smoke stained, I wouldn't worry too much about washing the wall before painting. If you paint cabinets in a kitchen scrubbing THEM down is important but kitchens are greasy smokey places oftentimes.

I only wash walls if they're greasy. Otherwise, a microfiber cloth will remove all the dust. And then paint away!

Unless the walls are really dirty I wouldn't work about it too much. You can dry dust them to knock off any dirt, but honestly use a good primer and a washable paint for the future.

Hi Susan, Here's a link that may help: https://homeguides.sfgate.com/make-flatpainted-wall-washable-45651.html

Good luck!

The key is a "damp" cloth, not soaking wet. Going over the flat paint surface with a damp cloth should work just fine. You might want to use a primer or paint/primer combination if you are concerned with new paint adhering to the wall. Hope this helps!