Measure the windows to determine how much molding you'll need. Gather all the required tools and clear the room so you can set up a convenient "shop." Roll up area rugs or cover carpeting with canvas drop cloths. Use plastic drop cloths to cover any remaining furniture. Set up a worktable, or plywood on a sawhorse, for your saw. Set a large window fan on "exhaust" to draw the dust out of the room and away from your work.
Always take the necessary precautions when working with a saw. Remember to wear goggles or safety glasses.
To avoid mistakes, make this your mantra: measure twice and cut once. Take your time and work on larger windows first. That way, if you do cut a piece of trim too short, you can use it on a smaller window.