If the painting has been framed behind glass, then the first thing that you need to do is to remove it from the frame. Obviously, it is better to photograph your pastel painting before you have it framed in the first place, if you can!
Mount the painting taught and flat on a stiff board - secured with some masking tape around the edges.
Site your painting in good, well-diffused light - preferably natural sunlight - in order to bring out the best of the colors.
If you have a tripod, set your painting-up so that it is perfectly perpendicular to the camera. If you don't have a tripod, just site the painting in such a position that you can hold the camera perpendicular (or 'square-on') to it without straining yourself! This is very important. You need the camera to be 'square-on' to the painting - otherwise, your image will be distorted.
With or without a tripod, your camera also needs to be pointing at the center of the painting and not located off to one side. So, perpendicular to the the surface of the painting and located so that it is pointing at the center of the thing!
If you have a tripod, or if you can borrow one, it will allow you to use a longer exposure, without getting any camera shake. This, in turn, will mean that you will be working with a smaller aperture - thus helping to keep the whole painting well focused.
You can even get away with photographing the painting behind glass if you use a polarizing filter but, you will always get a better photo if the painting is not framed behind glass.
Oh... And don't forget to crop your photo so that it shows only the painting itself... We don't really need to see half of your furniture or the color of your wallpaper!!
Edited by smb