simple make a room, put any fixture in it, select the fixture and assign it to a pawn like a bed, when a pawn uses a private bathroom they will get a mood buff like with dining rooms, however there is also a bed link button to link it to the nearest bed to inherit its assignment, but in the current version it doesn't technically set the rooms status to private, it only functions for the restrictions, thats fixed in the next version and the manual assign has been removed leaving just the bed link button to simplify the process

The other benefit is that if a pawn has a private bathroom then you know they always have access to a toilet or shower to use even if all public bathrooms are in use, and another benefit in the next update is that after pawns wake up they are much more likely to use the toilet and shower for a short time, so if they have private toilet and shower or bath then they can use it straight away leaving them free for the rest of the day to work with fewer interruptions

Oh, the bed link was not working for assigning private bathrooms, that explains the lack of the mood buff.
But about public bathrooms, there is any point on making them impressive?
I have had one for quite a while, and didn't see any negative or positive thought about them

I used to have private bathrooms for every colonist and making it impressive. Its space consuming, and for me, its not worth it. A pawn will only stay for a short while in a toilet and shower and longer for the bathtub. Some pawn have a "cramped interior" temporary debuff if they stay too long (when they are really filthy) in my 1x2 bathroom(bathtub). No debuffs from 1x1 shower and toilet. The positive mood buff for shower and bathtub are the same, but bathtub adds a little recreation. 1 tub and 1 toilet for every 2 colonists is more than enough for their needs.

For short, there is no need for the bathrooms to be impressive since there is no positive buff specified for bathroom impressiveness(space and beauty still matters on occasion). On another note, if you use stall doors, everything on the other side of the bathroom stall door is considered as bathroom since it does not separate rooms (NOW you can have your impressive bathrooms if it is connected to an impressive room e.g. dining/ rec room, careful though as that area will be re-labeled as bathroom. lol).

im doing tweaks in the next update to make pivate bathrooms a little more useful