Measure the size of the picture. Typically, you need a mat that is around 2 inches wider on the top, left and right sides and 3 inches wider on the bottom. However, do take into account that the mat is going to cover up about an 1/2 inch all the way around the side of the picture. So if you have a picture that is 11-by-18, you will want a mat that is 14-by-22.
Determine the size of your picture frame. It is sometimes easier to pick a frame out and cut the mat to fit the frame. Most frames only come in set sizes, and anything different is going to be more expensive because a professional framer needs to build the frame.
Decide what main color of mat you want. The ideal mat is going to push the focus of the viewer towards the painting. With this in mind, you should look away from bright colored mats, such as red, yellow and orange. Unless the picture is full of these colors, the mat is just going to distract the viewer. A black mat is always a good choice because the color recedes and send the viewers eyes onto the picture.
Decide if you want a different color for the cut of your mat. When a mat is cut, the edges pointing towards the picture are roughly 45 degrees, leaving a sharp edge against the picture. Often times the color underneath the display color of the mat is white. This can add an interesting accent to the mat, without drawing attention away from the picture. This is more of a stylistic preference and doesn't have much of an influence on the viewer.