Flooring is pivotal to all prayer rooms as people from many faiths tend to sit down or kneel while offering prayers.
It is advisable to use plush carpeting in your prayer room. Not only are they comfortable to sit on, but also keep heels from clacking, thus reducing noise, and making the room more quiet.
Also consider using cushioned mats to increase the comfort.
Go for soothing walls
This is a personal as well as a religious choice.
The color of a room's walls help set the mood of the area.
So, make sure you paint the walls of your prayer room with such colors that you find calming and uplifting.
Also, consider turning to your religion for color symbolism. You can add that color as much as you'd like.
What about the altar?
Since an altar is the focal point of your prayer room, you need to go right about it.
The size of an altar depends on how much you expect to place on it, so consider that.
However, simple surfaces such as a coffee table, or a dresser or a china cabinet would suffice. You may consider giving it a fresh coat of paint.
Install (minimal) furnishings and accessories
Furnishings: Do not go overboard. For your prayer room, it is best to stick to the necessary furnishings only. Consider installing a cabinet/chest of drawers, a book stand for keeping structures and a couple of lamps.
Accessories: Hang framed posters, add religious symbols, flower vases, baskets for offerings, and other religious items that you think should be a part of your prayer space.