If you don't measure correctly the first time, you'll have to make another trip to the store and buy more glass. Most stores have a no-return policy on cut glass. All window frames will expand or contract due to the weather elements.
Remove the broken glass from the frame and dispose properly. Wear a pair of leather gloves. Keep a piece of the glass to take to the store to match the type and age of the glass. Measure the thickness of the glass if you cannot take a piece with you to the glass store. The thickness will indicate the age and type of glass that was broken.
Use a screwdriver or small pry bar to remove the wooden or metal sash that holds the glass in the window frame. Clean glazing, adhesive or any other objects with the flat portion of the screwdriver or bar. Dispose of the small shards carefully.
Measure the full interior width of the frame opening to the nearest 1/16 of an inch. Record the size at the top and then at the bottom with the pencil and paper. Note any size differences between the two measurements.
Do the same for the overall height of the opening on the right and left sides. Again note any differences on the paper in the measured size.
Subtract 1/8 inch from the smallest dimension on both the width and vertical measurement to allow for expansion and contraction of the frame. The frame will swell in warm and wet climates and contract in cold climates. Properly measuring the glass will ensure a correct fit and proper seal.