Padding a piano bench is easier with two people working together, one holding the fabric in place and the other stapling. Loosely woven fabric can tear much more easily when stapled to the bench. Have the fabric match the style of the room. Usually a piano bench is not what you want to be the focal point of the room, so make the fabric blend in rather than attract attention. Sometimes it works to use the same fabric as throw pillows or curtains.
Durable fabric is a good idea.
Thicker fabrics work best, especially if your piano gets a lot of use. The padding is also a matter of preference, depending on the softness or look that you want the bench to have. About a one inch batting works well. If the top of the bench can be removed from the bottom portion, it is best to remove it. The top of the bench can usually be removed easily by unscrewing the two hinges attaching it.