How to ND a Window

What is ND gel used for?  Why would you want to use it on a window?  What are two key factors in setting it up?

    An ND, or neutral density, gel is used to lower the exposure of either an area on camera or a light source itself. You would want to use it on a window as windows often appear blown out when cameras are properly exposed for a subject that is indoors. Thus, you can use an ND gel to lower the exposure of the window and bring back details that were initially lost. Two key factors in setting up an ND gel are taping and squeegee. You should first go outside and check if it is possible to simply gaff tape the gel onto the frame or window. When you gaff the gel, make sure that the tape goes beyond the window and to stretch the gel as much as possible to prevent warping due to wind. With the squeegee method, you should first clean the window with windex, then spray more before applying the gel. After that, you spray more windex before squeegeeing as much off from both behind and in front of the gel. There may be specs of dirt that you can scrape off later.