This is, by far, the easiest one to do. All you need to do is try to get everything in that room to be the same color. Have a red table? Drop an apple on it. Plus a red lighting fixture, paint the wardrobe red and have it placed near your bed with the red blanket on it. Unless you’re colorblind (not dogging them, I just fully understand it’s not the easiest method for everyone) this should be the easiest one to do as long as you have the proper furniture.
This one is more end game as it requires you to have a wealth of DIY recipes and materials but if you’ve played the game since launch it might be possible at least with the basic block wood furniture.
For this one, you need to own every piece of furniture in one set. All of it. From the tables and chairs to the weird things like toys and various plumbuses. Place them all nicely in one room and receive a huge bonus.
Speaking of complete sets, an easy way to get a complete set is through seasonal items. Almost everyone who played during the Bunny Day event has enough eggs to have made all the furniture. If you can place them all in one room, having them being season items nets you an even bigger bonus. For example, this bonus grows even more if you can have a Bunny Day themed room during the Bunny Day event. Same with the Cherry Blossom stuff.