Summarize the following:
Close the door and measure along the top of the frame using a tape measure. With the door still closed, measure along both sides of the frame with a tape measure.  Note that your measurements for the top and sides of the door will need to be made along the door frame and not the door itself. To achieve a tight seal, you will need to measure cut pieces of weather stripping that are tailor fit to each side, which means that you will need to know the precise length of each side. Open the door, then measure the bottom of the door using a tape measure. Unlike the measurements you took for the top and side seals, you will need to measure for the bottom seal by measuring the bottom of the door itself. Make sure that you face the inside of the door as you take this measurement. This is the surface that you will be applying weather stripping onto. Use a tape measure to measure out each length onto the weather stripping for the top and sides of the door that you purchased. Mark the length of the bottom of the door on the door sweep weather stripping that you purchased.  Mark each length using a sharp pencil or marker. Make sure that each line you draw is clear and sharp. When using a weather stripping kit, you will have two long pieces for the sides and one short piece for the top. Make sure to draw your top measurement on the short piece and your side measurements on the long pieces. Cut the weather stripping at the marks you just measured out. Keep your cuts as clean and even as possible to ensure a tight seal. You'll also need to make sure that the ends of the top piece are angles and one end of each side piece is angled to fit into the top piece of weather stripping. You do not need to angle the bottom end of your side pieces.  The foam and vinyl portions can be cut using sharp scissors, but you will need a hacksaw or similar tool to cut any metal or wood portions. Hold the metal or wood still with one hand and saw through it with the other. Saw slowly so that you cut it in a straight line.

Summary:
Measure the top and sides of the door frame. Measure the bottom of the door. Mark those measurements on the weather stripping. Trim the weather stripping down to size.