Clean the side of the window where you are attaching the leading. Use glass cleaner and newspaper to remove all film, oil and debris from the window.
Open the window. Tape the pattern you want to the side of the window opposite the side to be leaded.
Flatten out the leading tape in your fingers. Do not stretch it. Start the leading along the edge of the window frame or glazing. Following your pattern, press the lead to the glass using the bone tool. Cut the lead to fit the window edges. Rub down the center of the lead tape, then use the end of the bone tool to press down the edges.
Cross over the lead tape at joints. If you cut the tape, you may create unattractive ends when you complete one side of the window. Clean the other side and lead tape it to match.