To begin with, we will start with the foil method. Now, if you have a frame that doesn't have any glass in it, and you can easily move it around... place the frame on a piece of white paper on a hard surface (the piece of paper must be larger than the frame), the front side of the frame has to be facing you. Before doing so, prepare the frame by cleaning out any old caulk in the frame, if needed. The paper can be plain or it can have your pattern mostly drawn out on it. With a pencil, trace around the inside of your frame, where it touches the paper. Now you know the exact size of the interior of the frame you are using. You can now draw your stained glass window to fit in that frame!
Next we will discuss how to do this with a leaded window, or a foiled window with a zinc or lead came border. First follow all of the instructions for the foil method as listed above. Then measure the width of your lead came from the outer edge of the came to the interior side of the came where it touches the glass in your window. Ok, what that means is, looking at the drawing below, measure from the outer area that is marked 9/32" to the interior section that says 5/32". The area between the arrows, drawn below.
Once you get that measurement (it should only be a fraction of an inch), draw another new line completely around the inside of the first traced line, that will be the width of the measurement you just calculated. Use the first traced line as your reference. Now the area of your pattern will be slightly smaller than the original window opening, as to allow the width of the came you are using as a frame.
If you are using a frame like in a door or house window that you can't remove for this tracing process, measure the area where you intend to place the window. Once measured, we like to draw our pattern about 1/32 to 1/8 inch smaller than the opening. Keep in mind that if you are using a came border to follow our paragraph above.
We advise new stained glass artists to practice fitting a window into a frame before agreeing to do this for a customer.