depends on your window. if the window is of wood construction and is a single pane [not a double with a gas in between] replacement is easy and is about a 5 on a scale of 0-10 in difficulty. you need certain tools to accomplish this task. a box of glazer points, a fairly stiff putty knife 1 1/2 wide, a can of DAP glazing compound, linseed oil, a small paint brush. Use gloves and remove as much large pieces of glass from the frame. Use a propane torch to soften existing glazing compound on the frame and don't burn the wood. Use the putty knife to remove the compound. Remove any glazing points from the frame. Remove any compound used for a "bedding" from the frame. Clean the rabbetted portion of the frame down to the wood. "Paint" some linseed oil into the rabbetted frame and let it soak in. Measure the frame minus 1/16" all around and order a piece of glass. Use your compound and put a small "roll" of compound in the frame. this is used to bed the glass. Place your glass pane into the frame and gently press all around. Then use new glazing points and press into the wood at about 8 inch intervals all around. Now make a larger "rope" of compound and press into place in the frame with the putty knife. Go all around the window. Now hold the putty knife at about a 45 degree angle and pull it along the frame and glass to form a bevel all around the glass. butter up the corners so it looks "professional." wait a day and paint the window. Be careful, the compound will be soft.