Typically you divide the large pane into two or three vertical sections with dividers between them. Each section can have three or four panes depending on your preference. Measure the original glass pane and draw a full-scale picture of the glass door on a piece of plywood; then draw your grid pattern on it. Alter the drawing until you get the pattern you're looking for.
Use 3/8-inch hardwood strips to build the grid pattern. The strips, also known as muntins, will divide the large piece of glass into what appear to be individual glass panes. Cut the strips to length according to your drawing. The best way to build the grid is to use a half-lap joint, which consists of notches cut into the contact points of the horizontal and vertical pieces. Cut the notches 3/16-inch deep on each piece with a table saw. Fit the notches together with glue to provide a stable, strong joint. Assemble the grid using the notches to make a pattern flat on both sides. Finish it with stain and lacquer and it's ready to place on the door. The grid is light enough to install using only silicone glue on all the contact points where the grid meets the framework of the door.
If you're not handy with woodworking, you can have plastic or nylon grids custom-made to fit your door. If your door is a standard size, premade grids may be the answer. Premade grids come in two types. One has a perimeter, which you screw to the existing frame on the window. The other type is made of individual muntins without a perimeter frame. You typically glue this type to the existing frame by dabbing silicone on the ends of the muntins. Another option is using a metal grid, which appears more substantial and can be ornate or simple, depending on your taste. Black wrought-iron creates a medieval look; anodized aluminum is available in a variety of colors to provide contrast. Or you can use metallic paint to draw a grid of panels on the window.
The most extensive option is to remove the original glass pane and replace it with a grid of individual pieces of glass in a framework the same thickness as the door. Start by building a grid using 1 3/4-inch solid hardwood, the typical thickness of most exterior doors. Nail strips of molding around the inside perimeter of each grid opening. Drop the glass panels against the molding and secure them with another strip of molding nailed tight around the glass on the opposite side. If you're investing this much expense and labor, you might as well upgrade your glass. You can install beveled or cut-glass panes or even stained glass. Remodeling a door this way will create a piece much more beautiful than average doors.