Measure, Then Order the Glass
Measure the area to be glazed, write down those measurements and take them to the hardware store. When you order the glass, you need to be as precise as possible with the dimensions.
At the hardware store, pick up a package of push points -- little triangles that will hold the pane in the frame. You'll just push those points right into the sides of the muntin bars. For a small pane of glass, use two points on each side of the glass around all four edges of the pane.