Purchase a leaded tape kit that includes scale paper patterns of Tudor-style window grilles. If the kit pattern is not full size, you will need to enlarge the pattern at a local photocopier store and print it out on large-format paper.
Clean the inside of the window with soap and water, followed by a glass cleaner. Dry the window with coffee filters so that no lint is left on the window.
Tape the pattern to the outside of the window and fit the pattern exactly the way you want the leading to look.
Use an artist's knife to cut the edge of the lead tape to the angle you need. Remove the backing paper and press the lead tape to the lines of the pattern. Where one tape crosses over another, cut across the diagonal, and remove the extra pieces of lead so that you create a tight mitered seam. Use a ruler or small straightedge to keep the tape very straight.
Roll over the fresh tape with a hard roller or boning tools to make sure the tape adheres well. To create an authentic look, go to the outside of the window and clean and tape that window to match the inside leading. You can use your pattern many times, and enlarge and reduce the pattern for odd-sized windows. Look at Tudor home photos for ideas on how to adjust the pattern for the best appearance.